What causes hot temperature extremes?
Within that galaxy cluster, things become chaotic as these galaxies are flung around but kept in orbit by their strong gravity. And as Steven Linton mentioned, 


  the galaxy's organized motion is turned primarily into disorganized
  motion within the material that is stuck together, which is what we
  know as heat. So the atoms of gas in these clouds collided and bounced
  off in all directions, where they hit other atoms, until the two
  clouds had formed one, with huge random internal motions (i.e. very
  hot).