You might wonder why anyone would announce that they had found nothing. Isn’t science about discovering things? Finding nothing is pointless because you can’t prove a negative, right? Not quite. It actually depends on the type of nothing you’ve found.
When we left our story we, had two proposed models: MOND, which posits that for very small forces, the acceleration of an object doesn’t quite go to zero, and dark matter, which introduces an invisible “something” that makes up most of a galaxy’s mass. Neither one of these seem particularly appealing, so why (as I stated yesterday) do I favor dark matter as a solution?
Both MOND and dark matter were introduced to address a problem with the way galaxies behave. At a basic level, the stars in a galaxy such as ours orbit the galactic center in roughly circular orbits. The speed of a star in its orbit should be governed by Newton’s law of gravity. So, using Newton’s gravitational theory, we can predict a star’s speed given its distance from the center and the distribution of matter in our galaxy. Newton’s beautiful theory doesn’t agree with the experimental data.
When we look at our galaxy, we find it has more mass than the matter we see. Much more. This is the first clue that there must be dark matter in the Universe.
The first evidence of dark matter came from observing the motion of stars in our own galaxy. It turns out stars were moving far more quickly than they should. Ever since then astronomers have tried to determine just what dark matter is.