University of Toronto, Canada To understand how much stuff there is in the universe, it helps to first understand what kind of stuff we are talking about. We currently believe that only about 5 per cent of the universe is made up of "ordinary" (or "baryonic") matter, which is things like protons and neutrons - the stuff that you, me, our planet and the stars, for example, are made of. The remaining 95 per cent comprises "dark matter" and "dark energy". But how do we know that only 5 per cent of the universe is composed of ordinary matter, and how much "stuff" does that actually translate into? One of the ways astronomers have been able to determine the contents of the universe is by observing the cosmic microwave background (CMB). This is like a snapshot of the early universe when it was only about 370,000 years old, and marks the moment when photons, or particles of light, were first able to travel freely throughout the universe.
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