Why is it not assumed that ‘black matter’ is simply the remnants of the Big Bang?

Answer

You must distinguish between ‘dark matter’ and ‘dark energy’. The first is matter that we cannot see (it does not interact with light or other electromagnetic radiation), but which we seem to find indirectly anyway: after all, from the movements of visible matter we see that there is much more mass that attracts than the one that we can see. Dark energy is the (confusing to me) name given to a mysterious force that accelerates the expansion of the cosmos. After all, we initially expect that the gravity of all that mass will slow down the expansion, but indications have now surfaced that an accelerating effect also occurs.

The big bang is indeed not finished yet, because the universe continues to expand. Gravity has not yet succeeded in bringing them to a halt and possibly turning them over. By the way, it seems that without dark energy, expansion would never stop. But what we see now is an acceleration. That is an additional effect that is operational today, and therefore cannot be attributed to the Big Bang.