Warming isn't just a terrestrial problem, it turns out - the entire universe is getting hotter.
But that doesn't mean we should give up fighting climate change, with scientists saying the two phenomena are unrelated.
Because of how long it takes light to travel across the universe, astronomers have been able to look back in time 10 billion years, and compare temperatures of gas and matter in faraway galaxies (in space and time) to those a lot closer.
They found the average temperature of matter 10 billion years ago, before the Earth was formed, was only one-tenth as hot as it is now.
"As the universe evolves, gravity pulls dark matter and gas in space together into galaxies and clusters of galaxies," said Ohio State University's Yi-Kuan Chiang, lead author of the study.
"The drag is violent - so violent that more and more gas is shocked and heated up."
In the early days of the universe, stuff was more spread out - over time gravity pulls it together. It's now just shy of 2 million degrees Celsius, on average.
While human activity has warmed the Earth, our efforts are nothing on an intergalactic scale.
"These phenomena are happening on very different scales," said Dr Chiang. "They are not at all connected."
These measurements are of the universe's matter, most of which is gas and plasma. Including the mostly empty space, it's -270.4C - barely above absolute zero, when all motion ceases.
The study was published in the Astrophysical Journal.
