Almost everything around us is made out of matter rather than antimatter. And this is not just true for our solar system or our galaxy, but appears to be true for the entire universe, or at least the observable universe. But except for having opposite charges, matter and antimatter are virtually identical. Then why should everything we see in the universe be made up of matter rather than antimatter? For that matter (pun intended), why is there any matter at all? Why shouldn't there have been an equal amount of matter and antimatter that all annihilated with each other and left nothing?
It is thought that matter and antimatter were formed in nearly equal amounts during the birth of the universe, but there was an extremely tiny excess of matter over antimatter, about one in ten million. Almost all of the matter and antimatter annihilated each other, and only the tiny excess of matter remained to form almost everything we see in the universe today. The process through how this tiny excess of matter was generated is called baryogenesis, which is still a mysterious process where speculation abounds.
Mark of Cosmic Variance is writing and editing a series of excellent posts about baryogenesis. Starting off with an introduction to the problem, so far he has talked about the theories of electroweak baryogenesis and leptogenesis. They are based on speculative theories of particle physics that have not yet been confirmed experimentally, which can be exciting for particle physicists since it means that they still have plenty left to do.
Whatever the mechanism, it must have happened uniformly across the universe after inflation, so mechanisms that are predicted to generate random amounts of excess matter or antimatter cannot be the vehicle for baryogenesis. Mechanisms for baryogenesis will have to take advantage of miniscule asymmetries between matter and antimatter, asymmetries that will have to arise in theories of physics that have yet to be discovered and confirmed.
One question about baryogenesis I had for the longest time was why it did not account for the existence of dark matter. If only the tiny excess of matter forms everything we can see in the universe, wouldn't the energy from the annihilation of the rest of the matter and antimatter be more than enough to account for the gravitational pull of dark matter? It turns out that I did not consider that light, which is the by-product of matter-antimatter annihilation, becomes less energetic with the expansion of the universe because the wavelength is stretched. This means that the energy density of light in the universe falls much faster than the energy density of matter, so gravitation due to light becomes negligible quite early in the life of the universe.
Where did all the energy in the light go? In a sense, the tens of millions of times more energy that used to be in light compared to matter literally all went into thin air; that is, into space-time itself. You could say that it all went into the gravitational energy stored by space-time as the universe expanded. The only caveat is that defining gravitational energy is a quirky thing to do in general relativity, which is the best theory we have so far for explaining gravitation.
Finally, an extra treat with a video from NASA showing the evolution of the universe since the time of the light-dominated period of the universe, when light had a significant gravitational influence, up to today, when light is no longer a major source of gravitation: