What is dark matter?
Question asked by: knowitall
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This is matter that, according to certain modern theories of cosmology, makes up 90–99% of the mass of the universe, whilst around 85% to others.
It's existence was postulated when it was noticed that normal visible matter did not have enough gravitational pull to keep the stars at the edges of galaxies in their place. With just conventional matter, they should fly out into space. Therefore, there had to be some unseen mass that was creating the extra gravity. The matter causing this effect is called dark matter.
It remains so far undetected, and may contain particles that are not yet known. It shows just how little we know about the world, that such a massive proportion of the particles in the Universe are a complete mystery to us.
There are millions of dark matter particles around us all the time, passing straight through us, and even the whole earth. Dark matter is almost metaphysical in that we cannot observe it directly, as it does not interact with us and is not made of atoms like visible matter.
As usual we are faced with stark possibilities, including:
- our current measurements or understanding of the amount of conventional matter in the universe is wrong
- there is dark matter that we know little about and may never know much about
- our current measurements re visible matter are correct, but the gravitational force is not fully understood and is not uniform, or perhaps other forces exist we are unaware of. Perhaps on the massive scale of galaxies the gravitational pull is somehow greater than the sum of its constituent individual masses, to the extent we do not need to postulate dark matter. If spacetime curves over large distances the messenger for gravity (gravitons?) may have a stronger effect than over shorter distances
- our current measurements are correct, but the theory of dark matter is wrong, incomplete, or not enough (e.g. add in dark energy and perhaps other types of matter too)
- some hybrid of the above
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