how did we discover dark matter?

Question: how did we discover dark matter?
- Keywords:
  dark matter
Asked by Jennifer Strutt to Daniel, Hannah, Maggie, Ry, Scott on 7 Nov 2017.
- Scott Melville answered on 7 Nov 2017:
  
  There’s a few measurements which point to dark matter. I think the first ones were of galaxy rotation curves. Imagine you were to take a yoyo, and spin it around really REALLY fast. Unless the string is really strong, eventually it’ll break and the yoyo’ll go flying. This shows that you need to provide really strong pulling forces if you want to keep things spinning really fast. When we measured how fast galaxies were spinning, we found that there’s no way the gravity from just their stars (and stuff we could see) could possible be strong enough to hold it together. So people made up ‘dark matter’, which is just extra stuff that doesn’t emit light (so we can’t see it from Earth), but adds some extra gravity to big galaxies so that they can spin without falling apart.
  Another way that we know dark matter is there is literally by looking VERY closely at pictures of big galaxies – particularly when they crash into each other:
  
  The Bullet Cluster – A Smoking Gun for Dark Matter!
  
  After a collision (called a ‘galaxy merger’) all of the stars and dark matter rearranges itself throughout space – but because they’re both pulling on each other gravitationally, we can trace out where the dark matter should be sitting 🙂
  But as to what this dark stuff actually IS, we have no idea – there are some experiments searching for it here on Earth, but no one has found it yet (it doesn’t seem to interact with anything – so it’s hard to build any kind of ‘box’ which you could catch dark matter with!).
- Maggie Lieu answered on 7 Nov 2017:
  
  The first discovery of dark matter was by a guy named Fritz Zwicky, he was looking at galaxies orbiting around each other in the Coma cluster of galaxies. When he calculated the velocities of the galaxies, they were travelling very fast. The galaxies should have been flying off in all directions because when he added up all the mass from the stars within the galaxies there was not enough gravity to keep those galaxies bound in a cluster. He called this ‘Dunkle Materie’ or dark matter, but it wasn’t really popularised until many years later an astronomer called Vera Rubin saw a similar effect with stars orbiting within a galaxy
- Ry Cutter answered on 8 Nov 2017:
  
  Trick question, we haven’t discovered dark matter!
  We’re pretty sure there needs to be more mass in the galaxy that we can’t see (see Scotts and Maggies answers), but we still don’t know where it is or what it’s made of.
  Some scientists don’t even think dark matter is real! These scientists think that we just don’t understand gravity enough. They use something called Modified Newtonian Dynamics (MOND) theories. None of them really work though, but the maths is very interesting.
  
  Great question,
  
  Ryan
- Hannah Middleton answered on 11 Nov 2017:
  
  We haven’t found out what it is yet! But like the others said, there is lots of evidence that dark matter is there as we can see the effects of it in space.

Question: how did we discover dark matter?

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