Back when astronomers first realized that our universe is expanding and not static, they began calculating the expansion rate. They measured as best they could how rapidly everything was flying away from everything else. Admittedly, their calculations were based on a few hard numbers and a lot of estimates. There are local areas in which galaxies are actually approaching one another instead of moving away. Distances and velocities are, at best, approximations.
As equipment improved and the ability to take more measurements and take them more precisely increased, our estimates of the masses of galaxies, their distances from us and each other, and the rate at which the universe is flying off in all directions became ever more precise and astronomers became more confident in their results.
And then everybody that studies these matters became puzzled. The universe is expanding, but it is expanding considerably faster than the calculations indicate. The hypothesis was formed that there is something called dark energy (it is termed “dark” not because we cannot see it but because we do not know what it is). The idea is that there is “something” that is causing the universe to expand faster than it would otherwise. That “something” has been dubbed “dark energy”.
Nobody has a very strong conception of just what this dark energy really is. Some have referred to Einstein’s Cosmological Constant. Others call it “quintessence”, which is a dynamic quantity that varies in time and space. The Cosmological Constant is also referred to as vacuum energy. Nobody really knows much about dark energy or how it manages to do what it does. It is estimated to constitute about 75% of the entire mass-energy of the universe, and we have no idea what it is.