The two biggies: Dark matter and dark energy.
We have proof that something weird is going on but we have no evidence of what it is and so we call it “dark” which is just a fancy way of saying “I dunno.” Some genius thought of using the word “dark” for something not understood, forever confusing the common public.
First, dark matter. This is a hypothetical kind of matter we know nothing about. Galaxies rotate, but they are rotating too fast. Stars in the outer parts of the galaxies should be flung out to space but they’re not because of too much gravity. In fact, scientists calculate that over 95% of the matter in the universe is dark matter, but we can’t see it or touch it or detect it directly and we don’t know what it is.
Second, dark energy. Astronomers have discovered something weird. Distant galaxies are moving away from us at an accelerating rate. The further away they are from us the faster they’re accelerating. Something is “pushing” them away and continues to “push” so they keep getting further away faster and faster. Reverse gravity? It’s not like we once thought, there was a big bang, the universe exploded into what we have now, and eventually gravity will slow everything down and it will fall into itself and crunch back into another big bang. No. Galaxies are not just moving away, they are moving faster and faster as if they had help from something. We don’t know what that something could possibly be so we call it “dark energy,” short for “I dunno energy.”
Scientists report what is observed and those observations are then corroborated by other scientists. The universe isn’t under any obligation to behave the way we expect it to behave and we don’t always know why. In fact, we may never know why, but we have observed the effects of both dark matter and dark energy, so scientists feel confident they exist even though nobody knows what they are.