Zwicky spotted that galaxies in clusters are whizzing round too fast, which could only be explained if there were a bigger gravitational tug on them. That bigger tug comes from dark matter. There’s another possibility that some astronomers favour: maybe our theory of gravity is wrong! Maybe gravity is just tugging those galaxies harder than we think. If this is the case, then the gravity theory has to be a lot more complicated.

Our current un-tweaked gravity theory gets the movement of planets in our Solar System right, and gets laboratory gravity measurements right too. So, if there are tweaks to the theory of gravity, then they must only be significant on very big scales and yet be negligible within our Solar System, and that inevitably makes the theory more complicated.

It would also mean that Einstein’s theory of gravity is wrong, and that some beautifully simple principles underpinning his theory that would be wrong too. However ‘beautiful’ and ‘simple’ certainly don’t mean ‘true’, and some astronomers think it’s preferable to tweak the gravity theory, instead of supposing that most of the matter in the Universe is something we can’t see or touch.

There are aesthetic pros and cons to both approaches, but a couple of experimental developments have made it difficult to avoid at least some dark matter in the Universe, even if you’re prepared to tweak your gravity theories.

One development is the measurement of structures in the Cosmic Microwave Background, which is the light from when the Universe was last opaque. There were sound waves around at that time, and the whole observable Universe was acting like a resonating cavity!

The remnant of these sound waves is still visible as structures in the Cosmic Microwave Background. The harmonics or overtones of these sound waves depend on what sort of fluid is sloshing around, and it turns out that the overtones are exactly what you’d see in the presence of dark matter.

The other development is the observation of two colliding galaxy clusters, shown in the image below. In this collision, the gas in the two clusters crunched up at the centre of the collision, and heated up. This hot gas gives off X-rays, seen in pink in the image. The dark matter on the other hand doesn’t interact with normal matter or even with itself, so it passes through the centre and out the other side.

The presence of dark matter can be inferred from the warping of space it causes, detected via the distortions the warping gives to background galaxies. This is shown in blue in the image. It’s quite hard to reproduce these data unless there’s at least some dark matter involved.
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