Long ago the universe was filled with neutral hydrogen, and not much else, a period known as the Dark Ages. Slowly, stars and galaxies were born and began to 'energise' this neutral hydrogen, and over time the universe became clear, as it is today. Looking deep deep deep into space, we can see the light arriving to us from incredibly distant objects. In 2011, a team detected light from a quasar (an incredibly bright and energetic galaxy) 13 billion light years away. This is a measure of how long it took the light to travel to us (13 billion years). Quasars emit their light particles (photons) at many different energies, and as these photons travel through space to arrive at us, they can be absorbed by gasses or materials they pass through on their journey. Each material will only absorb photons that have a certain energy, and because of this we can know what things the photons passed through based on which energies are missing. The light from this incredibly distant quasar was missing the energies that correspond to neutral hydrogen. This tells us that this quasar is so ancient it was formed during the dark ages of the universe, and its light began the journey to Earth in the cloudy young universe.
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