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Dark Matter Measurements
ksp0704 writes: "According to this article at space.com, scientists have finally measured the approximately 90% of the universe we can't see (the dark matter)." I'm sure it will continue to be a topic of debate for years, but two independent measurements agreeing is a good sign.
They look at the galaxies, and estimate how many stars and stuff there is in the galaxy. Any rotating galaxy. And They figure out how fast the galaxy is rotating.
They notice a problem. For any rotating galaxy there is not enough star stuff to hold the galaxy together. The spiral arms should never be there.
The star stuff in the galaxies do not have have enough gravity to hold galaxies together. Galaxcies should not exist at all. Stars should be all flying about because that is how weak the gravity is.
Just how much too weak? The Star stuff has one tenth the gravity needed to do the job. so something has to be doing the other 90%.
That is what the dark matter is. It is a term to label what the other 90% is. The don't know what it is yet. but they are working on it.
"It is a greater offense to steal men's labor, than their clothes"
We have three methods to calculate the mass of the universe. Two are based on electromagnetic interactions. Those two agree. The other is based on observations of gravitational interactions. It gives a result 10X as large for the total amount of matter. Therefore 90% of the universe is made of particles that interact gravitationally but not electromagnetically. The only way to observe them is to observe their gravitational effects. Like, duh. Why is this such a difficult concept to grasp? It's an empirical observation.
Keep in mind that if something only couples gravitationally, it's going to be extremely hard to see. You're prejudiced by your own experience with the world, which is mostly based on electromagnetism- meaning interactions with photons (real and virtual). Get rid of electrodynamics, and most concepts and phenomena you're familiar with- atomic physics, chemistry, biology, optics, materials science, friction, pressure, radiation, viscosity, resistance, reflection, transparency, iridescence, impenetrability- all this stuff goes out the window! Your ass would sink through your chair, right through the ground, until you reached the center of the earth with everything else. Don't underestimate the importance of photon-mediated interactions. Everything else is gravitation, beta decay, and the strong nuclear force. Of those three, only gravitation operates over non-microscopic distances. And it is very weak. There could be up to several tons of dark matter in the room with you right now. You would never know it's there.
Of course, the mass could be ordinary matter that we're just not seeing. Many people like the idea of lots of Jupiter-sized objects. Lots of black holes might also work (although a black hole can feed off either kind of matter).