In today's image, we see a cluster of galaxies. Nearly every object in this image is a distant galaxy. When we measure the mass of galaxy cluster like this one, we find that there is a large amount of dark matter needed to explain the motions of the galaxies and the gravitational lensing caused by the cluster. Dark matter makes up the vast majority of the matter in the universe.

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This Podcast is an authorized New Media Mirror Site for APOD. You can access the image and website directly at: Astronomy Picture of the Day. You can use the archive link below the image to select the photo corresponding to the date of this podcast.

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