I see in the news today that there is new evidence that "dark energy" is real. Here is what I think I understand about dark energy:
Since the Big Bang the universe has been expanding. Given that the Big Bang was one heck of an explosion it seems reasonable that matter is still flying outwards in all directions, but it isn't just matter -- space itself is expanding outward. (Actually it is space-time that is expanding, but including mention of time complicates what I have to say without adding anything). It is easy to understand how ordinary explosive energy propels matter about, but harder to see how it can even get a grip on space unless it is by way of the grip out going matter has on space through gravity. It seems to me now that it is not even indirectly the normal energy of the Big Bang that accounts for space expansion but the mysterious dark energy that does it. Moving matter, far from causing space expansion, is being dragged along by it and at an accelerating pace as dark energy continues to accelerate the expansion of space. As the force of dark energy works over distance to accelerate matter, as well as space, dark energy is being converted to ordinary kinetic energy. Dark energy must be a lower entropy form of energy than kinetic energy. I believe it must be the ultimate lowest possible entropy form of energy. It must be the "prime mover" of energy -- the kind that creates other kinds of energy but was itself never created from anything else. I further believe matter itself was created, in nearly equal matter-antimatter quantities, from dark energy not unlike the way the energy of a gamma ray can create an electron-positron pair. Some matter-antimatter creation from dark energy is likely still going on. This echoes somewhat the old "steady state" theory of the universe that was superceded by the Big Bang theory.
Dark energy is not a mere oddity of cosmology. Our lack of knowledge of it is a hole in our understanding of physics as fundamental as fundamental can be.