I have yet to hear a good explanation of dark energy. But then again, Einstein called entanglement "spooky action at a distance" in 1930, but the paradox was explained by John Bell in 1964, and just recently demonstrated. There's still time for further research.

Dark energy comes from cosmology. It's a theoretical construct. We can observe the expansion of the universe by making measurements of the velocities of things that are very far away. The faster most things on opposite sides of us are moving away from each other, the faster the universe is expanding, the faster things move toward each other, the faster the universe is contracting. If the averages balance out, the universe is remaining the same size. (This is a gross oversimplification, but there's only so much one can do with one paragraph.) Based on the current theory of gravitation (from general relativity), we can take the age of the universe, the size of the universe, and the speed at which the universe is expanding, and back-calculate the acceleration. But the calculation doesn't make sense from our current understanding, because things are much more stable than they ought to be. If you think of matter, it becomes less dense as it expands, because density is mass divided by volume. However, if gravitational force is the only thing binding the universe together, there is a critical density, above which the universe will implode, and below which the universe will explode. But, since things are too stable, physicists have come up with the idea of something (enter: dark energy) that has density that is not a function of volume. That thing keeps the density of the universe more constant (but not entirely constant) as it expands, allowing matter to have plenty of outward motion, but preserving the force binding the universe from flinging apart from itself.

Personally, (I have an MS in Physics, but the people who came up with dark energy are way deeper than I am) I would suggest that, maybe, there is another force or geometry for which we simply have not accounted by general relativity. About 130 years ago, people thought that there was an all-penetrating ether, and Einstein's Special Relativity shifted the paradigm to explain why that construct was not necessary. Dark energy, to me, looks like a similar construct to the ether, just with some fancier mathematics involved, and, of course, on a much bigger scale. We already know that forces work on completely different scales from one another, so perhaps the force of gravity is not the largest scale force, and we just don't have enough data nor observational power to "see" whatever the next level of force is. But then, in General Relativity, gravity isn't seen as a force so much as a quirk of the geometry of space on a large scale. To get from regular geometry to the kind of geometry in which gravity pops out naturally, you have to play around with positive and negative signs of things in an interesting way. At the time when Minkowskian geometry was formulated, it was surreal and bizarre for pretty much everyone. Maybe the reason we cannot explain the expansion of the universe is simply because it is so weird and foreign to us that we have not yet had enough time, as a society, to think about a solution.

Prove it!

Space travel will increase now that its being handled by the private sector instead of the government. And I evidently have more faith in the human race than you do! I'm fairly certain our children will solve the challenges that face them before the entire human race is annihilated.

Lol, I said dark energy, not methane!