Dark Matter
Dark Matter and Dark Energy are both fudges to make observations match what we think we know.
A solar system follows the rules. Further away stuff has a much longer orbital period. A spiral galaxy ought to be the same. Instead the outer bits rotate nearly at same rate, as if it's a giant solid disc (like how a record or floppy spins). This seems to contravene Kepler's laws (originally Newtonian stuff) as well as Einstein's stuff.
Also the rate of expansion of the observable universe seems wrong (viewing galaxies), considering mass and energy.
Dark Energy and Dark Matter make the the macro scale of the Galaxies (our own and others) and the observed universe fit the known "laws" and physics.
So one solution is finding out what Dark Energy and Dark Matter are, the other possibility is that on a Macro scale something else is happening. Adding an extra non-inverse square law term to "gravity" doesn't seem to work well and what is the theory anyway?
The observations of the expansion have been revised, due to interstellar dust and gas reducing photon/EM energy and thus making red shift seem larger than it should be. More dust and gas has been found in the galaxy, but not enough to account for the lack of difference in orbital period of outer and inner parts of spiral arms.
It's a really interesting detective story with both space observation (I look forward to James Web) and lab tests including LHC.
Biting the hand that feeds IT
