The Cold Spot in the cosmic microwave background radiation has led astronomers to speculate the possibility of parallel universes.
The question isn't are we alone in the universe, it's are we alone in these universes? According to a bit of the Cosmic Microwave Background Radiation (CMB) we might have a neighbor.
The CMB is the leftover light from the Big Bang itself. It's the furthest thing from us in the visible universe. It's all around us constantly, but it's super low-energy, because, again, it's been around since the beginning of the universe. Over time, we've taken the CMB's temperature, mapped it in increasing detail and in doing so we found something curious.
See, usually the CMB is pretty uniform in a temperature of about 2.73 Kelvin. Except in one place: The Cold Spot.
For some reason there it skews colder than the average temperature by a few microkelvin. No big deal for a tiny area, but this is consistent across a huge region! It's a billion light-years across! This spot challenges what we know about the universe. Astronomers have been fighting about The Cold Spot for years, and they've come up with a few theories...
The most banal theory is that it exists because the satellite instruments or analysis has some math error.
But the most fantastical is also now one of the most likely: that the spot is cold because it's where the edge of our universe touched the edge of another universe!