Astronomy Test 23

Why do we call dark matter "dark"?
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What is the ultimate fate of an open universe?All matter decays to a low-density sea of photons and subatomic particles.What is the evidence for an accelerating universe?White-dwarf supernovae are slightly dimmer than expected for a coasting universe.If there is no dark matter in the Milky Way Galaxy, what is the best alternative explanation for the observations?Our understanding of gravity is not correct for galaxy-size scales.If a galaxy's overall mass-to-light ratio is 100 solar masses per solar luminosity, and its stars account for only 5 solar masses per solar luminosity, how much of the galaxy's mass must be dark matter?95%Which of the following is an example of baryonic matteryouMeasuring the amount of deuterium in the universe allows us to set a limit onthe density of ordinary (baryonic) matter in the universe.What does the universe look like on very large scales?Galaxies appear to be distributed in chains and sheets that surround great voids.What might be causing the universe to accelerate?We don't know!-but we call it "dark energy."