I thought it was "dark" because it was harder to detect than gravity. No?
Well, there's a couple of "dark" things being bandied about in modern astronomy/cosmology.
Dark Matter: Dark matter is the unseen mass that surrounds galaxies, or clusters of galaxies. When observing a galaxy's shape and spin as the stars orbit their common center of mass, they orbit in a way that's
impossible for the amount of mass and gravity that the visible stars represent. (example, imagine you're on the moon, observing a satellite in orbit about the Earth, but it's speed for that given orbit is such that the Earth would have to be 2 or 3x as heavy as you know it must be for the satellite to be orbiting in the way it is. So something is creating "extra gravity".)
So same with the stars moving about their galaxies, there has to be some kind of extra dark matter/mass, something we can't see, that's exerting the extra gravitational pull to make the galaxies retain the shape they do, and the individual stars we can see orbit in that galaxy the way they do.
The other proof of this that we see, is that galaxies create gravitational lensing (light from further objects bending around the galaxy) we can observe that is also out of proportion to it's visible mass.
There's several different theories, some partially proven as to what this extra dark matter is.
- More regular matter that's not lit like stars so it's hard to see, brown dwarves, way more rogue planets that don't orbit stars than we ever thought existed, black holes that have nothing nearby to suck in and give away their presence, gas dust... We've found some new categories of these things, however, none of them have come even within an order of magnitude of providing the extra mass.
- Swarms of exotic "slippery" particles that don't have mass and exert gravitational influence, but don't interact with other particles or matter. Like a neutrino but "heavy".
- Some fundamental misunderstanding, or heretofore unobserved macro effect of space-time and gravity.
Dark Energy: Most who are somewhat conversant with astronomy and cosmology know all the observed galaxies and clusters are flying away from each other, and just like motes of dust in an expanding balloon, the farther points of dust are receding faster than the closer points.
However, as astronomers have measured the rate of which the farthest galaxies are receding, they're actually
speeding up. Which was quite a shock. So instead of just moving away faster the farther you look, they're
accelerating, as if they're being pushed by
something.
Possible answers include that gravity is actually a repulsive force over ultra-long distances, or again, some unknown new force in the universe, or some fundamental misunderstanding of motion, gravity, and space-time over very long distances and very large scales.
