actually, the amount of dark matter in the universe is more like 25%. The funny thing about dark matter is that it is sort of like looking for a black cat in a coal cellar in the middle of the night without any lights. There are no known methods for detecting dark matter.
The reason that we believe that it exists is due to gravitational forces: Observable matter like stars and stuff like that just do not generate the amount of mass needed to hold a galaxy together. Consider how sand slips through your fingers instead of gravitationally binding together, and how the Earth is a 6 sextillion ton ball of rock that holds itself together despite the gravitational pull of the sun and the moon. Going by regular matter alone, galaxies should fly apart like sand. Something is holding galaxies together. We call it dark matter because we assume it to be a form of particle that we are not yet able to detect. Some scientists even believe that general relativity breaks down on a galactic or extragalactic scale, much the same way general relativity breaks down on a quantum scale, which is why we are not able to calculate why the galaxies are holding themselves together.
So in a nutshell, the reason we know dark matter exists is through indirect observation of galaxies, and that percentage is the amount of dark matter (in relation to regular matter) needed to hold a galaxy together.
I mentioned dark matter being about 25%, with conventional matter being about 5%. The remaining 70% is an even more mysterious phenomenon: dark energy. In the 1990's scientists observed something baffling: the universe not only seems to be getting bigger, but it seems to be accelerating. Galaxies are moving faster away from each other now than they did a billion years ago. Calculations in order to accelerate the growth at this rate indicate that 70% of the universe must be made up of this dark energy.
The reason that we believe that it exists is due to gravitational forces: Observable matter like stars and stuff like that just do not generate the amount of mass needed to hold a galaxy together. Consider how sand slips through your fingers instead of gravitationally binding together, and how the Earth is a 6 sextillion ton ball of rock that holds itself together despite the gravitational pull of the sun and the moon. Going by regular matter alone, galaxies should fly apart like sand. Something is holding galaxies together. We call it dark matter because we assume it to be a form of particle that we are not yet able to detect. Some scientists even believe that general relativity breaks down on a galactic or extragalactic scale, much the same way general relativity breaks down on a quantum scale, which is why we are not able to calculate why the galaxies are holding themselves together.
So in a nutshell, the reason we know dark matter exists is through indirect observation of galaxies, and that percentage is the amount of dark matter (in relation to regular matter) needed to hold a galaxy together.
I mentioned dark matter being about 25%, with conventional matter being about 5%. The remaining 70% is an even more mysterious phenomenon: dark energy. In the 1990's scientists observed something baffling: the universe not only seems to be getting bigger, but it seems to be accelerating. Galaxies are moving faster away from each other now than they did a billion years ago. Calculations in order to accelerate the growth at this rate indicate that 70% of the universe must be made up of this dark energy.
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