Our reality relies on balance and symmetry. From our physical laws ("for every action there is an equal and opposite reaction") to philosophical musings ("as above, so below") through pop culture ("the love you take is equal to the love you make"), the idea that nature is balanced by default seems hardwired into our heads. And it is, for the most part. The symmetry of the laws of nature holds up pretty well, up until you get to one of the biggest unsolved problems in all of cosmology: there's a whole half of the universe that's straight up missing.
The current model of our universe suggests that for every particle of matter, there should be an equal particle of antimatter. Of course, that isn't true, evidenced by the fact that we're still here. If antimatter was in equal parts to our boring old regular matter, the whole universe would explode. Our model of the universe states that there should be an equal amount of antimatter, yet it's only through antimatter's mysterious absence that we have a physical universe at all. So what gives?
A new theory may have the answer. According to an article published in the journal Physical Review Letters, the antimatter problem may be solved if we consider our universe after the big bang as only one half of the universe. The other half of the universe would be a mirror image of ours, stretching backwards in time from the big bang and composed almost entirely of antimatter, just as ours is composed almost entirely of matter. This mirror image would have all of it's properties flipped: positive charged particles would be negative, up would be down, left would be right. The same as ours, just opposite in every way.
This theory has several advantages, says physicist Neil Turok. For one, it explains another sticky problem of cosmology: the existence of "dark matter," which makes up the bulk of the universe yet is invisible and undetectable. This mirror image model of the universe states that all that elusive mass from dark matter may be caused by elusive, ultra-massive particles called "sterile neutrinos," which would have a mass some 500 million times that of a proton. Interestingly, this number would line up with some anomalous readings taken by the Antarctic Impulsive Transient Antenna (ANITA).
Another advantage is that it gives us a way to explain several of the complications with the period of rapid expansion right after the big bang without having to invent new particles or forces.
A problem with the theory, however, is that it doesn't explain some of the temperature variations in the cosmic microwave background radiation, leftover energy from the big bang, that scientists have detected. According to Neil Turok, that was a sticking point with the peer reviewers for the paper, but he says:
“They said you have to explain the fluctuations and we said that is a work in progress. Eventually they gave in.”
Whether or not they gave in from being convinced that they were in fact working on it, or were just tired of a physicist's browbeating is unknown, but all new theories generally have problems like this. They're trying to explain the universe, after all. It's complicated stuff.
It's certainly an intriguing theory, and the model of a universe with two equal yet opposite halves extending in in either direction through time is a beautiful and elegant model. Is it true? We'll see. But don't worry, even if this does explain ours, we'll have a whole other universe to be wrong about.