The idea of dark matter in the universe dates back to the 1930s, with the observation that the gravitational force on the visible matter in clusters of galaxies could not fully account for their behaviour, implying some alteration to gravity, orthe existence of non-luminous, invisible matter. Now a team in the US has used a combination of astronomical images to analyse gravitational lensing in a region where two clusters are merging. The researchers find that their observations cannot be explained by modified gravity.
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