In the beginning there was only darkness and empty space.The big bang was space dividing into matter and anti-matter. The net sum of the universe is still zero.
Many people believe that the big bang was caused by a sphere of extremely high mass exploding. However if that were so, then where did the mass come from initially. Also if this were true, there would have to be a center of the universe, but scientists say that the universe has no center.The material we now call "dark matter" is not matter at all. It is anti-matter. It undoubtedly emits anti-photons, but when they collide with matter they are destroyed. Also anti-matter has anti-gravity with respect to matter. That is it repells matter and it is the force of repulsion that is causing the universe to expand.
If the universe were only made of matter, when a black hole is formed it would continue to grow and eventually its extreme gravity would suck the entire universe back to the highly concentrated sphere of matter. That doesn't happen because the black hole sucks in anti-matter which destroys an equal amount of matter, thereby limiting the growth of a black hole.
I suspect that the basic particle of matter is the photon, which at rest is mass and in motion is energy.
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