Measurements of the heat left over from the Big Bang suggest there are about 1090 photons in the Universe – making them around 10 billion times more abundant even than hydrogen atoms. But while photons are the most numerous known particle, the Universe also contains so-called ‘dark matter’, whose composition is a mystery.
One possibility is that it’s made up of particles called axions, whose mass may be so low that they could only account for dark matter by existing in larger numbers even than photons.
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- Have the first dust particles from interstellar space reached Earth?
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- What if the Big Bang was not the beginning?