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The Milky Way and Andromeda galaxies, along with about fifteen or sixteen smaller galaxies, form what's known as the Local Group of galaxies. The Local Group sits near the outer edge of an enormous supercluster, the Virgo cluster. What's more, the Milky Way and Andromeda are moving toward each other, the Local Group is falling into the middle of the Virgo cluster, and the entire Virgo cluster itself, along with a second supercluster, is speeding toward some unfathomable mass known only as "The Great Attractor."
These structures and their movements--which can't be explained as part of the expansion of the universe--must be guided by the gravitational pull of matter. And yet, scientists have not yet detected enough matter to account for this tremendous gravitational pull. And so we must add one more player into our hierarchical scenario: dark matter.
Ever more powerful instruments have made it possible to look farther out and farther back than ever before possible, and what cosmologists are seeing is simply astounding. Theories about the origin and fate of the universe are being variously bolstered, refined, or turned on their heads based on these new observations.
Cosmologists hope that their observations will help answer these questions, some of which have dogged scientists for centuries:
In the following sections, we'll examine what and how cosmologists know what they do know, and some of the "burning issues" in cosmology.
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