Dark Matter May Feel a “Dark Force” That the Rest of the Universe Does Not

After decades of studying dark matter scientists have repeatedly found evidence of what it cannot be but very few signs of what it is. That might have just changed. A study of four colliding galaxies for the first time suggests that the dark matter in them may be interacting with itself through some unknown force other than gravity that has no effect on ordinary matter. The finding could be a significant clue as to what comprises the invisible stuff that is thought to contribute 24 percent of the universe.