The thing is a bit more complex.

We have a theory of the big bang,which could not be disproven yet.

This theory would expect that our current universe would slow down in expansion.

However our observations show that expansion is accelerating.

Introduce dark energy. It is supposed the mystical force which expands the universe.

(not sure if I get the following right) This force acts on every atom. But it is weak on its own, even the weak gravitational force is stronger. However over great distance the force is bigger than the gravitation.

Then there is dark mater. Our visible universe was made into a simulation where they use all the physics we know of to simulate the currently visible situation. However the simulation did not behave like what we see. It only works when they added a lot of matter. But matter that you can’t see? “dark matter”

Then they formed new theories that both dark matter and energy needed to exist or our universe wouldn’t look like it looks today.

Now I am no scientists but it is possible that we are completely wrong here.