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Does Dark Matter?
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MACHOS
 
Some of the invisible dark matter that is missing from the Universe may be massive dark bodies such as planets, black holes, asteroids or failed stars (brown dwarfs). These don't produce light and so cannot be seen over great distances. [We can see planets and asteroids that lie within the Solar System because they reflect light from the Sun, but they don't make their own light like stars].
All these heavyweight candidates for the missing mass are collectively known as 'MACHOs', Massive Compact Halo Objects. This is because they are large bodies that live mostly in the 'halo' of galaxies. This is like the 'outer suburbs' of the galaxy, extending from outside the 'inner city' of the galaxy's core right to the wastelands at the edge.
Another possibility is that 'dark galaxies' may exist - mini-galaxies that roam undetected in intergalactic space. Around a thousand times smaller than the Milky Way, these galaxies would be too faint to see from Earth. Instead of harbouring billions of stars like our own Galaxy, they would be full of brown and black dwarfs.
As with all dark matter, dark galaxies and MACHOs are both hard to detect, as we can't see them. But a surprising solution comes from the amazing effect that gravity has on the fabric of space. Everything with mass has a gravitational field, even you and me. The bigger you are, the more gravity you wield. According to Einstein's theory of General Relativity, gravity actually bends the fabric of space and the more gravity an object has, the more it warps space. This was proved by Arthur Eddington on an expedition to the Amazonian forest in 1919.
This effect can be used to detect MACHOs. When a really massive object [such as a MACHO] moves in front of a distant star, its gravity bends the space around it. When light from the distant star travels through this distorted region of space it bends and becomes magnified. So while the MACHO is crossing in front of the star, it seems to move and grow in size. This is known as gravitational lensing and allows astronomers to weigh the invisible MACHO causing this phenomenon.
There is no doubt that MACHOs account for some of the missing dark matter in the Universe. But there is a limit to the amount that can be attributed to them alone. This is governed by the amount of atoms formed in the Big Bang, putting a limit on the amount of ordinary matter there can be in the Universe. All in all, MACHOs could account for around 20% of the missing dark matter needed to glue the Universe together. To solve the rest of the story we have to start looking for matter that isn't ordinary, but 'exotic', such as WIMPs or neutrinos.
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