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THINK of a galaxy and you probably think of stars, billions of them, in
brilliant shining spirals. Or perhaps you picture nebulae鈥攙ast clouds of
glowing gas. But these are just a tiny part of the whole. What galaxies are
really made of is dark and slippery. And it now seems that it鈥檚 also very
strange.

When astronomers realised several decades ago that most of the galaxy is made
of matter we can鈥檛 see, they came up with a cosmic menagerie of possibilities
for the invisible matter, including massive black holes, microscopic black holes
and 鈥渟hadow stars鈥� that only interact with the world through their gravity.
Eventually, two candidates took the lead: MACHOs (massive compact halo objects),
which are failed stars, too dim to see, and WIMPs (weakly interacting massive
particles), which are exotic particles that interact only reluctantly with their
surroundings.

For years, the battle raged between MACHOs and WIMPs, but by the early 1990s
it seemed that MACHOs had finally won the day. Now everything has changed.
MACHOs are down and out, and exotic WIMPs are in the ascendant. The stuff of the
galaxy, it seems, is anything but ordinary.

So what originally led astronomers to believe that we are surrounded by
bizarre galactic ectoplasm that outweighs all ordinary matter? It鈥檚 all because
our Galaxy spins faster than it should. Except near the galactic centre, stars
and gas are circling at an average speed of roughly 200 kilometres a second.
Even far out, beyond our already suburban neighbourhood, where the Galaxy thins
out and the rotation should be much slower, stars keep the same impatient pace.
If the Galaxy were only made of the things we can see, it wouldn鈥檛 exert a
strong enough gravitational pull to keep all these speedy stars orbiting.
Instead they would zoom off into intergalactic space, and our night skies would
be dull and empty.

This isn鈥檛 just a problem for the Milky Way鈥攐ther galaxies rotate too
fast as well. So astronomers were forced to admit that there is more to space
than meets the lens: our Galaxy, like most or all others, must be surrounded by
a halo of dark matter that dwarfs the small bright island of stars.

But what could this dark matter be? It needn鈥檛 be completely invisible, just
very dim. And a simple way for it to lie low is in so-called 鈥渂rown dwarfs鈥�:
stars that have less than a tenth of the mass of our Sun. This means they are
too small to ignite nuclear fusion in their cores, so they just glow very
faintly as they slowly lose the small store of heat gained in their
formation.

Could an army of brown dwarfs be holding the Milky Way together? These dark
lumps of ordinary matter, called MACHOs, can鈥檛 be spotted in the usual way.
Instead, you have to look for the imprint of their gravity, a phenomenon called
gravitational microlensing.

This ploy was suggested back in 1986 by Bohdan Paczynski, an astrophysicist
at Princeton University in New Jersey. If you stare at a distant star for long
enough, he reasoned, a MACHO should occasionally drift in front of it. The
MACHO鈥檚 gravity will slightly bend the path of light from the star, converging
it towards us and making the star briefly brighter. He also calculated that it
would be an incredibly rare event, so to catch an example of this refracted
glory astronomers have to watch a lot of stars at once. Millions of them.

Since the early 1990s, several different research groups have been doing just
that. They have chosen to watch stars in the Large and Small Magellanic Clouds,
two small galaxies that orbit the Milky Way. That way, there鈥檚 plenty of space
in between the Earth and the star being watched, increasing the chance that a
MACHO might pass across our line of sight.

Too heavy

Sure enough, in 1993, MACHO sightings started to come in. Over a few weeks a
target star would brighten and then fade, in exactly the way predicted by
Paczynski. Astronomers were delighted: it looked as though they might have
solved the mystery and found the main ingredient of the galaxy.

But the delight was short-lived. Most of the twenty or so MACHOs spotted
since 1993 are simply too heavy. You can estimate the mass of a MACHO from how
long the lensing lasts: the bigger the MACHO, the larger its gravitational
sphere of influence and the longer the brightening. And it turns out that the
MACHOs seen so far are much larger than expected, at around 0.3 to 0.8 times the
mass of the Sun.

This is a most uncomfortable number. If MACHOs are so big, what can they be?
They can鈥檛 be ordinary stars, because with masses much more than 0.1 times that
of the Sun they would shine, and the galactic halo would be bright instead of
dark. They might possibly be old white dwarf stars, the dying embers of stars
similar to our Sun. But if so, it is hard to explain why space isn鈥檛 full of
heavy elements, the waste products of these old suns.

Double trouble

There is a third possibility, which is a very troubling one for MACHO
hunters. In 1994 Kailash Sahu, now at the Space Telescope Science Institute in
Baltimore, suggested that the lenses might not be in our Galaxy鈥檚 halo at all.
Instead, he said, they might be ordinary stars in the Magellanic Clouds. If so,
the lensing objects could not explain the Galaxy鈥檚 missing dark matter after
all.

A hint that Sahu was right came with the discovery in 1996 of a very strange
lens in the direction of the Large Magellanic Cloud (LMC). Instead of the
simple, smooth, symmetrical rise and fall in brightness, this lens produced a
much messier fluctuation with very sharp peaks of brightness. Rather than a
single object doing the lensing, it looked as though two close
companions鈥攁 鈥渂inary lens鈥濃�攈ad come between us and the target
star.

It was a crucial finding, because it turns out that you can use the sharp
peaks, together with some complicated geometrical reasoning, to measure how
quickly a binary lens is moving across the sky. And the binary lens was
suspiciously slow. Rather than matching the speed of the hectic halo of the
Milky Way, it was moving more like something in the smaller, more placid
LMC.

But this event was only recorded by one telescope. With only these
observations to go on, the brightness variation wasn鈥檛 pinned down properly, so
MACHO champions weren鈥檛 too put out. Then, last June, another binary lens was
seen, in the direction of the Small Magellanic Cloud. On this occasion, it was
spotted in time for a host of telescopes to be aimed at the lens, catching the
second brightening in detail. Again, it moved far too slowly to be in our
Galaxy鈥檚 halo鈥攊t looked like a binary within the SMC. 鈥淥ut of fewer than
20 microlensing events towards the Magellanic Clouds, the only two that allowed
a distance measurement were both in or near the clouds themselves,鈥� observes
Paczynski. 鈥淚t seems very unlikely that most of the halo is in the form of
惭础颁贬翱蝉.鈥�

So MACHOs are looking badly bruised. And now a fatal blow may have come from
another quarter: the halo鈥檚 dark matter can鈥檛 be made of the kind of ordinary
particles that build stars, planets and brown dwarfs. Three months ago,
astronomer Dennis Zaritsky of the Lick Observatory on Mount Hamilton in
California reported that the Milky Way, and other galaxies, must contain
something more exotic.

Most of the lightest elements that we see around us in the
Universe鈥攄euterium, helium, lithium鈥攚ere made a couple of minutes
after the big bang, when the Universe was small and cosy, and neutrons and
protons were hot enough and close enough together to fuse into atomic nuclei.
Astronomers can estimate how much helium was made in this way by measuring the
light spectrum of vast intergalactic clouds left over from the big bang. They
can then work backwards and calculate how big a pool of protons and neutrons the
big bang had to work with, or in other words how much ordinary matter there is
in the Universe.

Knowing this number, Zaritsky decided to weigh the Milky Way and other
galaxies more precisely than has ever been done, to see if there was enough
ordinary matter to account for their mass. To do this, he drew together
observations of satellite galaxies of the Milky Way. About a dozen mini-galaxies
are in orbit around us, with the two Magellanic Clouds being the biggest and
best known. Their movements can be used to weigh a greater volume of the Milky
Way and its halo than would be possible by looking at those speedy stars in the
galactic disk. The most distant satellite galaxy, Leo I, is 700 000 light years
from the galaxy鈥檚 centre, thirty times as far out as the Sun and fifteen times
as far as the stars at the fringe of the galactic disk. Because its orbit
encloses everything inside that radius, Leo I can be used to weigh a very large
volume indeed.

Combining velocity measurements of all the satellite galaxies, Zaritsky
showed that our dark halo goes out at least as far as Leo I, and has a mass
equivalent of at least a thousand billion Suns, more than ten times that of all
the visible matter in the galaxy. What鈥檚 more, he discovered that other galaxies
that are similar to the Milky Way also have vast haloes. Adding it all up, there
must be far more matter in galactic haloes than there is ordinary matter. Haloes
must be made of something odder.

So, if the dark matter isn鈥檛 dim and ordinary, it must be invisible and
extraordinary. This isn鈥檛 as far-fetched as it sounds. We already know of a type
of matter that is invisible: neutrinos. These particles cannot interact with
light鈥攖hey can鈥檛 absorb it, emit it or reflect it. It just isn鈥檛 in their
nature. They can only interact with matter using the weak force, which is aptly
named, so they interact only rarely. And there should be huge numbers of
neutrinos left over from the big bang.

Coming unstuck

But neutrinos have drawbacks as dark matter. Though they probably have some mass
(This Week, 13 June 1998, p 25),
they move too fast to be easily captured
by the gravity of galaxies, so would not make effective galactic glue. Worse
still, they are fermions, a type of particle that according to quantum mechanics
is fundamentally antisocial: no two neutrinos in the Universe can share a single
quantum state. Remarkably, this means that even the vast spaces of many galactic
haloes would seem too cramped for neutrinos to be the dominant mass.

What astronomers need is an exotic particle that interacts only weakly, is
more sluggish than the neutrino and preferably isn鈥檛 a fermion. Fortunately,
physicists have already invented two particles that fit the bill.

The first is an axion, a hypothetical particle that interacts using only the
weak nuclear force, and was devised to help solve a minor inconsistency in the
standard theory of particle physics. Though axions are astoundingly lightweight,
enough might have been made in the big bang to explain all the dark matter in
the Universe.

In principle, axions can be turned into microwave photons using a strong
magnetic field, and so detected. It is a very delicate experiment, but a
collaboration in the US between the Massachusetts Institute of Technology,
Lawrence Livermore National Laboratory in California, The University of Florida
in Gainesville, Fermilab near Chicago and Lawrence Berkeley National Laboratory,
has built a detector that has already ruled out a small range of possible axion
masses. But the theory is vague: it only pins down the axion mass to between
about a millionth and a thousandth of an electronvolt (an electron, for
comparison, weighs half a million electronvolts). The part of this mass range
that would produce a useful amount of dark matter is between 1 and 10 millionths
of an electronvolt. 鈥淚 think that whole range will be probed with extraordinary
sensitivity within five years,鈥� says Leslie Rosenberg of MIT. Until then, we
can鈥檛 rule out axions as a dark matter candidate.

But the other candidate鈥攖he WIMP鈥攊s more promising still. WIMPs
come out of another theory designed to improve on standard particle physics.
Many physicists believe that to unify the strong nuclear force with
electromagnetism and the weak force, there must be twice as many subatomic
particles as we have yet discovered. This is the theory of supersymmetry. Each
familiar particle has a much heavier 鈥渟uper-partner鈥�: quarks have squarks,
electrons selectrons, photons photinos. Most of these particles are exceedingly
unstable, and quickly fall apart. But the least massive super-partner, the
neutralino, should be stable. And unlike the uncertainty over axions, there are
theoretical reasons for expecting them to be important in astrophysics.

鈥淣eutralino WIMPs are required by particle theorists to weigh about 100 times
as much as a proton,鈥� says physicist John Ellis of CERN, the European Laboratory
for Particle Physics near Geneva. And according to his latest calculations, they
should have been created in the early Universe in similar numbers to protons and
neutrons. 鈥淪o they should provide about the right amount of mass to be dark
matter.鈥� Bingo鈥攁 perfect WIMP. But can anyone prove that they exist?

Plenty of people are trying. Several teams have built WIMP detectors
underground, away from the highly radioactive environment of the Earth鈥檚
surface. If WIMPs make up the halo they must be drifting through the Earth all
the time, and occasionally one will bump into an atomic nucleus. In a salt mine
in Boulby, Yorkshire, a British group has set up experiments around several
large crystals of sodium iodide, a substance that should emit a small flash of
light when one of its nuclei is hit by a WIMP. The task is complicated by
background noise from the decay of traces of radioactive isotopes in and around
the crystal, which produces at least a hundred times as many flashes as WIMPs
could.

A cleaner technique, employed by the American Cryogenic Dark Matter Search
collaboration, is to use a crystal of germanium. If a WIMP hits a germanium
nucleus, the recoiling nucleus should heat up the crystal and ionise hundreds of
atoms. Both the heat and the ionised charge can be measured, and the ratio of
the two should distinguish a WIMP collision from most forms of background
radioactivity. 鈥淥ur detector will soon be competitive with the other
experiments,鈥� says Roger Dixon of Fermilab. And it has the potential to be more
sensitive still.

But one team, working under the Apennines at the Gran Sasso Laboratory in
Italy, has caused something of a sensation by announcing the detection of 鈥渁
possible WIMP candidate鈥�. At a cosmology meeting in Rome in October, Rita
Bernabei of an Italian-Chinese collaboration dubbed DAMA reported an intriguing
variation in the Northern-Hemisphere number of counts in their sodium iodide
crystals. Over the two years the experiment has been running, more flashes of a
certain energy have been seen during summer than in winter. This is just what
you would expect from WIMPs: in summer, the Earth is moving around the Sun in
roughly the same direction as the Sun is moving around the centre of the Galaxy.
The two motions add up, so the Earth would be moving relatively swiftly through
the galaxy鈥檚 cloud of WIMPs. About 10 per cent more WIMPs would pass through the
detector than six months later, when the Earth鈥檚 overall motion around the
centre of the Galaxy is slower.

Going to ground

It is only the fairly low-energy flashes that vary in this way, and from that
the team calculates that their WIMPs have a mass towards the bottom end of
expectations鈥攁bout 60 billion electronvolts, or roughly the same as an
iron atom.

So is the mystery solved? Not quite. Bernabei and her team only describe this
as a hint of a detection. The statistical significance is not overwhelmingly
high, and they can鈥檛 be sure that no other errors have crept in. Many WIMP
physicists consider even that to be too strong a claim. 鈥淚t鈥檚 like seeing a
light in the sky and saying you鈥檝e discovered a UFO,鈥� says Peter Smith of the
Rutherford Appleton Laboratory in Oxfordshire, a member of the British dark-matter
collaboration. 鈥淚t could be a WIMP signal changing through the year; but
it could also be the background changing. Instead of describing this effect in
terms of WIMPs, we need to do better experiments.鈥�

Bernabei counters that no seasonal variation is seen in the higher-energy
events in their crystals. 鈥淚f a modulation in the lowest-energy region were due
to a modulation of the background, an equal or larger modulation would be
present at higher energies,鈥� she says. But both agree that the question can鈥檛 be
resolved without more data, and every dark matter group has plans to develop
more sophisticated detectors.

Not all the detectors will be down mines. WIMPs should occasionally lose
energy when they hit nuclei in the Sun, and so become trapped by the Sun鈥檚
gravity. Over billions of years, a huge stock of WIMPs could have built up in
the Sun鈥檚 core. Occasionally these trapped WIMPs would collide with each other
and be destroyed, producing high-energy neutrinos that would zip out into space.
A few instruments for detecting such high-energy neutrinos are under
construction. One, called AMANDA, is at the South Pole and looks down at about a
cubic kilometre of ice for the distinctive patterns of light produced by
neutrino byproducts.

But there is a more direct approach: to make a WIMP. The Large Hadron
Collider, now being built at CERN, will be the most powerful particle
accelerator in the world and it may be able to create WIMPs. Particle physicists
will try to make a WIMP to prove that supersymmetry describes the Universe, but
as a fringe benefit they may also prove that WIMPs fill it.

Whether or not the main ingredient of the galaxy has been glimpsed under the
Apennines, the case for WIMPs has never been stronger. MACHOs have had their
brief hour of glory, but after the dust has settled WIMPs will probably prevail.