Cosmic strings are theoretical fault lines in the universe, defective links between different regions of space created in the moments after the Big Bang. And they might be theoretical no longer - distant quasars show the fingerprints of these strings.
The search continues for dark matter, the mysterious stuff that makes up the vast majority of all matter in the cosmos. Although nowadays they're most likely undetectable subatomic particles, in the universe's beginnings they could have formed super-explosive dark stars.
Global warming really isn't just a local problem...universal warming ran through the universe 11 billion years ago, doubling the temperature of intergalactic helium. The cosmic temperature spike was so bad, it stopped galaxies from developing for 500 million years.
We've always assumed our sun was the only star in our solar system, but maybe not. We could be in a binary system, with a brown dwarf hiding in the Oort cloud. And it could be bombing us with comets.