The soul always hears an admonition in such lines, let the subject be what it may
“Did you know that 95% the universe is dark matter?” (…) “What about dark energy?” (…)
It is true that most of the universe is made up of things that can’t be seen, and whose presence is inferred by its effects on the things that can be seen. But what effects do we see? And how does that translate to a percentage of the universe that remains a mystery?
Galaxies rotate, and when we use gravitational laws to predict what that rotation should look like, we find that they should behave like the solar system–the farther away something is from the central mass (in the case of the solar system, the sun, and in the case of a galaxy, the supermassive black hole at the center), the slower it should orbit; the gravitational force on Pluto, for instance, is much smaller than the gravitational force on Mercury, because it’s much farther away from the sun. A star at the fingertip of an arm of the galaxy should orbit more slowly than we do. However, that’s not really what happens: galaxies have flat rotation curves, meaning that objects farther away from the supermassive black hole don’t really orbit more slowly than things closer to it.
What this implies is that there is lots of mass spread throughout the galaxy–that most of the mass isn’t just at the center–and that the spread-out mass has a large gravitational effect on the galaxy’s rotation.
photos { 1. Pearly | 2. Natalia Arias }
