Correct we don’t have direct evidence from that early right after the Big Bang but we can use mathematical models we’ve developed from centuries from scientific experimentation. We then work these models back to conditions that we would expect that early in the universe. These models then make specific predictions that we can perform other experiments to see how accurate our models are. For example the standard model predicted that in conditions with very high temperatures the fundamental forces become one and at slightly lower temperatures something called quark gluon plasma would form where protons and neutrons themselves break down into almost a soup. When we were able to engineer particle accelerators to have enough energy we were able to actually produce quark gluon plasma and partially prove that at least the weak force and electromagnetic force combine into the electroweak force.

It’s all kind of like a big jigsaw puzzle. Every time we make a new discovery it’s like we add another piece telling us more and more about the nature of the whole picture. If you have 60% of the puzzle finished and you see a corn field it’s reasonable to assume the rest isn’t going to be something like an underwater scene.

We don’t know everything about what happened immediately after the Big Bang, but assuming the laws of physics weren’t simply different then it’s not going to be drastically different than what we think barring some new paradigm shifting discovery.