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DARK matter and dark energy are mysterious phenomena that have captured the attention of astronomers, cosmologists and particle physicists for decades and they both have the word 鈥渄ark鈥� in them, so they must be related right?

After my last column, a curious reader wrote to me to ask about the relationship between the two phenomena. I have decided to use this first column of the year to answer this question, which goes to the heart of some of our most fundamental concepts in physics and some of our biggest open problems in astrophysics and cosmology.

Dark matter and dark energy are related, but perhaps not in the way you might expect. For the most part, theoretical physicists and observational astronomers expect them to be different entities, and the majority of our research is organised around the idea that they have very little relationship to one another. Mostly what connects them is a tendency of 20th-century scientists to name things they can鈥檛 see or understand as 鈥渄ark鈥�.

To understand the difference, you need a grasp of what matter and energy are. Most people probably encounter these physics concepts through pop culture representations of Albert Einstein鈥檚 famous equation, E = mc². 鈥�E鈥� represents energy, the 鈥�m鈥� represents mass, and 鈥�c鈥� represents the constant speed of light.

Mass is a measure of roughly how much matter there is. We are most intuitively familiar with mass via gravity: here on Earth, our mass determines the strength of our gravitational interaction with the planet, something more popularly known as our weight. We understand that our weight roughly gives a measure of how much matter there is inside us.

Energy is a more abstract concept and developing an intuition for it generally takes time, even for students of physics. We can get some feeling for it from Einstein鈥檚 famous equation, which tells us that an object that has mass has an equivalent energy that is found by multiplying the mass by the speed of light squared.

Part of what makes Einstein鈥檚 relativity so powerful is that this equation shows that the traditional Newtonian distinction between energy and matter is unnecessary. When we combine this idea with the revelations of quantum mechanics, we find that where there is matter, there is energy, and where there is energy, there is the potential for matter to randomly appear.

鈥淲here there is matter, there is energy. Where there is energy, there is the potential for matter to appear鈥�

Because of this matter-energy equivalence, particle physicists tend to state measurements of both mass and energy in units of energy.

Relativity isn鈥檛 the only theory of physics that informs our thinking here. In Einstein鈥檚 theory, empty space isn鈥檛 predicted to have any energy in it, but in quantum mechanics, empty space (also known as the vacuum) will have an energy associated with it.

It is here that we run into what is often called the 鈥渃osmological constant problem鈥�. Quantum calculations predict that there will be an energy everywhere in space-time, even when there is no matter present. But astrophysical observations tell a different story.

In 1998, the expansion of space-time was found to be accelerating with time and the simplest theoretical explanation for this is the presence of a cosmological constant, a vacuum energy that is everywhere.

Importantly, because of the matter-energy equivalence, vacuum energy has the potential to interact with gravity in a manner similar to that of massive objects, except instead of acting attractively, a vacuum energy with a positive value will act like a pressure pushing outward, forcing space-time apart.

The exciting discovery of cosmic acceleration has also created a mystery because the amount of vacuum energy needed to match the data is much smaller than the amount of vacuum energy predicted by quantum calculations. This is known as the dark energy problem.

This is quite different from the dark matter problem. As I explained in an earlier column, the dark matter problem is to do with missing mass in galaxies (15 May 2019, p 26).

We think dark matter manifests as a massive object, while dark energy doesn鈥檛. Yet there is no law of physics that says dark matter and dark energy can鈥檛 be connected. Together they make up 96 per cent of the combined matter-energy in the universe, and it is natural to wonder whether they are related.

Over the years, physicists have considered this possibility, and along with two of my colleagues, I am now one of them. It is too early to say whether our model works. It may be that, in the end, the only connection between dark matter and dark energy is our ignorance about how they both work.