My understanding of dark energy is a little different. As I understand it, we figured gravity pulls things together, right? So everything should be kinda slowly falling back together from the big bang. It was theorized to end in a 'big crunch' where the universe collapses back and then explodes again in a cycle.

Only when they tried to measure how fast distant objects were moving relative to us, they found that things were still moving away from each other. More than that, the farther away things were, the faster they were moving. Meaning distant objects were accelerating.

Acceleration requires energy, but we don't know the mechanism behind this, or where the energy comes from. Hence, dark energy.