Recently I looked into the hottest(and coldest, but in another question) natural places. I find this to be a "cloud of searing gas that is surrounding a swarm of galaxies clustered together five billion light-years away in the constellation of Virgo". This place has a temperature of 300 million degrees Celsius, which is super hot even compared to the temperature at our Sun's core. A quick search why gives:
A group of galaxies collided violently with another swarm of galaxies at a speed of 2,500 miles per second.
However, what mechanism exactly causes this temperature? Friction? Or something else? Would stars there be fusing stuff from the outside?