Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. It is difficult to hide one galaxy far behind another. The closer galaxy's gravity will act like a huge lens , pulling images of the background galaxy around both sides. These images are more than sideshow s, since the distance between background images increases with the mass of the lens. This lens mass turns out to be much greater than the sum of all its stars - indicating the presence of dark matter .
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