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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.

“This result, if confirmed, could upend our understanding of dark matter,” says physicist Don Lincoln of the Fermi National Accelerator Laboratory in Illinois, who was not involved in the research. So-called “self-interacting dark matter” has been suggested for some time but it has generally been considered unorthodox. The simplest model of dark matter portrays it as a single particle—one that happens to interact with others of its kind and normal matter very little or not at all. Physicists favor the most basic explanations that fit the bill and add extra complications only when necessary, so this scenario tends to be the most popular. For dark matter to interact with itself requires not only dark matter particles but also a dark force to govern their interactions and dark boson particles to carry this force. This more complex picture mirrors our understanding of normal matter particles, which interact through force-carrying particles. For example, protons interact through the electromagnetic force, which is carried by particles called photons (particles of light).

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