Dark matter has always been a tricky thing to nail down. It was first proposed to account for the discrepancy between the measured rotational behavior of galaxies and the rotation predicted by the combination of Newtonian gravity and the visible mass. Over time, it became clear that the normal matter, which interacts readily with light, accounts for less than 10 percent of the matter in the universe. The remaining matter could be inferred based on the dynamics of galaxies and larger structures, but could not be seen directly, and so picked up the name dark matter. Some scientists, however, have not been convinced that dark matter must exist, and suggested that we might have an incomplete understanding of gravity instead. It has been difficult to resolve this conflict, as dark matter doesn’t interact with light, and its gravitational pull ensures that there’s always regular matter associated with it, confusing efforts to detect it. According to new observations, that last sentence should be rewritten to read ‘almost always.’ Using mutliple observations from a number of space-based astronomical platforms, a unique collision between two clusters of galaxies has been observed at a number of wavelengths. That collision, shown on the right, […]

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