Structures in the Universe
You may have heard of the "Dark ages" of history (500-1000 AD), but have you heard of the dark ages of the universe? This period occurred between 380,000 years to 200,000,000 years after the big bang. It gets its name from the fact that matter had cooled down sufficiently to form hydrogen and helium, but it hadn't yet clumped together into stars. Hence, without any sources of light, scientists only see darkness when looking back toward this period of the universe. To try and understand what happened during this period, scientists are now doing research on gravitational waves- a phenomenon that persists throughout almost the entire life of the universe.