I want to reveal one of the greatest mysteries in science to you:
Look at this gif, notice any patterns in the orbits? I’m talking about the speeds. The planets orbiting around the Sun get slower the farther out they go.
Why is this? Kepler’s Third Law of Planetary Motion.
The stars in the Milky Way orbit around the Galactic Center just as our planets do around the Sun. We find that the farther you get from the center of the galaxy… there’s approximately no change in orbital velocity.
Just like that, one of our most well-established tools in astronomy (Kepler’s Third Law) becomes ineffective for some reason.
It’s become apparent to astronomers that when something orbits around a center of mass, it’s orbital speed has a relationship with the distance from the center of mass.
Right now we don’t think most of the mass of the Milky Way is focused in the galactic center, but hidden in darkness surrounding the galaxy on all sides.
What we know is that it’s impossible to see and so far has eluded detection: we’ve started calling this mysterious mass dark matter.
