The universe is cold at 2.73 kelvin, which is -270.42 C (-454.76 F), but the small temperature it does have is very useful indeed. This radiation is called the Cosmic Microwave Background (CMB) and it was emitted only a few hundred thousand years after the Big Bang. In a universe that’s 13.8 billion years old, that’s extremely early. Over the eons the wavelength of this radiation has been stretched to the microwave end of the spectrum by the expansion of space.
The CMB tells us many things about the universe and we’ve used lots of high-tech instruments to analyse it. The radiation is homogeneous, meaning it’s largely the same in every direction we look, with only tiny fluctuations amounting to no more than about 20 microkelvins either way (and a microkelvin is 1 millionth of a degree).
That is, except for one particular cold spot, which is on average 70 microkelvins colder than the rest of the CMB and up to 150 microkelvins colder in some parts. This makes cosmologists rub their chins, mutter things like “hmm” and then they start penning theories as to why this might be.
Did we collide with a parallel universe at some point? Is it just a void of nothingness? Or did someone leave the window open when the early universe was forming?