What is Dark Energy? It’s a word we use to mean that there is more to learn about the nature of the universe.
The Big Bang Theory holds that the universe started from an infinitesimal point with nearly infinite energy that started to expand 14 billion years ago. There is evidence for this. For example, deep space is at about 3 degrees Kelvin. As the universe gets bigger, the energy is distributed over a larger volume, so is cooler.
The energy wants to expand, and gravity wants to pull it all back together. A big question in Physics was how the universe would end. Energy could win, gravity could win or it could be a draw.
Ending in the Big Freeze, meant that gravity was too weak to pull everything back together. The universe would continue to expand until the energy density is so low, that no life could exist and nothing else could happen.
The Big Crunch meant that the relentless pull of gravity would slow the expansion until it reverses, causing the universe to collapse. Then there could be another Big Bang, and it all starts over.
A third option was that the expansion would slow, but gravity wouldn’t be strong enough at such large distances to cause a collapse. The universe would remain in a steady-state. That seemed unlikely.
Better tools, like the Webb Telescope, yielded more and better information. We found that the universe is actually expanding faster. That wasn’t one of the conceivable options. There are four fundamental forces in the universe, and none of them can account for a universe that is getting bigger, faster. Physicists have no explanation, so new theories were needed. Dark Energy is like a word variable that stands for this thing we can’t explain.
There could be a fifth fundamental force, the four fundamental forces may be more complex than expected or numbers we call “constants”, may not be constant over long distances or time scales. Not comfortable for physicists, but it does gives them a problem to resolve.