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WHATEVER you do, don鈥檛 ask where it happened. 鈥淭he most common misconception is that the big bang was an explosion in a particular place,鈥� says , a cosmologist at the University at Buffalo in New York. 鈥淭hat鈥檚 just completely wrong.鈥�

The best evidence for the big bang is all around us in the cosmic microwave background, the radiation released once the universe had cooled sufficiently for atoms to form, when it was about 380,000 years old. And that is the point: everywhere in today鈥檚 universe was where the big bang was. 鈥淚t鈥檚 not something that happened somewhere, but something that happened everywhere, including the space you happen to be occupying now,鈥� says Dan Hooper at Fermilab in Illinois.

When cosmologists talk about the big bang, they are talking about an extremely dense, hot state that existed around 13.8 billion years ago and which has since expanded and cooled to make the universe we know today. Extremely dense and hot 鈥� but not infinitely so.

The idea that the universe was created from an infinitesimal speck, known as the big bang 鈥渟ingularity鈥�, comes from winding the showreel of an expanding, cooling universe backwards and not stopping until we get to a beginning. But, unfortunately, our current theories of physics can鈥檛 deal with space and time on such unfathomably small scales. So we can say nothing sensible about the moment when the universe was a single point, if indeed it ever happened. 鈥淲e may just have to come to terms with that,鈥� says Kinney.

鈥淭he big bang didn鈥檛 happen somewhere, it happened everywhere鈥�

However it all started, the big bang that followed wasn鈥檛 so much an explosion, which implies stuff flying off randomly, but a perfectly uniform expansion 鈥� albeit one that seems to have been, in its first throes at least, unimaginably fast. We have suspected this since the 1980s, when we realised that the temperature and density of stuff in the universe 鈥� as seen for instance in maps of the cosmic microwave background 鈥� are incredibly smooth. We would naively expect quantum fluctuations to have produced regions with differing densities and temperatures in the universe as it spread out. To explain the observations, physicists invented a split-second growth burst called inflation that stretched the universe so fast the kinks were smoothed out.

Standard timelines of the universe鈥檚 first instants have inflation coming after the big bang. But for Kinney, it makes more sense to think of inflation coming first, creating the hot, dense soup from which today鈥檚 universe, with its atoms and stars and planets, emerged through a slower process of cooling and expansion. 鈥淲hat we think was the beginning of the universe 鈥� this hot equilibrium state 13.8 billion years ago 鈥� was in fact the end of inflation,鈥� says Kinney. If that jars, then Hooper recommends thinking of inflation鈥檚 鈥渂ig stretch鈥� not as something separate, but as part and parcel of the big bang.

Then again, not everyone buys the inflation story, not least because it isn鈥檛 clear what did the inflating. Some cosmologists prefer to think that the universe began with a bounce, when a previous universe contracted to an extremely hot, dense point where it could contract no more. This would not only do away with the singularity, but also answer another inevitable, yet largely inscrutable question about the big bang: what came before it.

Cutting-edge science throws up all sorts of controversial, nebulous and mind-bending concepts. Here鈥檚 your guide to how to think about some of the fiddliest of them:

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• Dark energy: Understanding the mystery force that rules the universe