• Big Bang nucleosynthesis: Observations are consistent with model for all light elements, agree with independent measurements, and suggest normal matter makes up only a small fraction of the energy density of the universe
• Helium-4: Mass fraction determined by ratio of neutrons to protons, since uses up almost all of the available neutrons
• Deuterium: The more neutrons/protons there are flying around, the less deuterium survives (b/c it gets converted, in steps, to He-4)
• Radiation/matter: Density of matter drops with size cubed (volume), but radiation loses by an additional size factor due to cosmological redshift. Universe switched from radiation to matter dominated around 70,000 years after BB

• Freeze out: As universe cooled, photons lost energy and so couldn't convert back and forth between matter and radiation anymore
• Recombination: Cool enough for electrons to combine with ions to form neutral atoms, around 300,000 years after BB
• Surface of last scattering: Before recombination, light scattered off electrons; after, streamed freely (except for absorption lines). Can see back to these "clouds" looking at cosmic microwave background
• Reionization: First stars plus quasars emitted strong UV radiation and reionized their surroundings

Hanny Van Arkel, 25, came across the strange gaseous blob while using the Galaxy Zoo website to help classify galaxies in telescope images. Astronomers subsequently confirmed that the object was one-of-a-kind.

[...] "The quasar itself is no longer visible to us, but its light continues to travel through space and the Voorwerp is a massive 'light echo' produced as the light strikes the gas," Dr Chris Lintott, from Oxford University, explained.