re: Dark matter and dark energy.

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Our equations don't match reality, so we propose a form of matter that cannot be interacted with in any way and whose only effect on the universe is to neatly and exactly fill in the gap in said equations?

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Actually, there's indirect physical evidence for both dark matter and dark energy. When the great astronomer Vera Rubin assessed the rotation of galaxies about their centers, she noticed that the outer-most stars orbited at the same speed as those closest to the center.

This is counterintuitive, of course, and her discovery startled the physics community. The inner-most stars should orbit the center much faster than those further out, similar to the orbits of our planets about the sun. The only sensible explanation is that the great gravity needed to hold 100 billion+ stars in such a rigid dance about the galaxy's center is coming from an unseen and, so far, undetectable source of matter.

Other indirect evidence for dark matter is gravitational lensing. Many distant galaxies cannot be seen directly from earth because of other galaxies sitting between us and them. However, concentrations of dark matter on our line of sight to some galaxies exerts tremendous gravity upon the light from them and actually bends their light around the blocking galaxies and we see distortions of them. See illustrations below.





Indirect evidence for dark energy is the ever increasing rate of expansion of the Universe. It was Edwin Hubble in the 1920s who first determined that the Universe was expanding.

It wasn't until the 1990s, however, that astronomers determined that the expansion was occurring at a constantly increasing rate. The sensible explanation is that a force is pushing matter apart faster and faster. The term dark energy just means that we have no clue what it is. Or dark matter, either, for that matter (pun intended ).

It's thought that dark energy overcame the pull of gravity some 5 billion years ago and began to express its domination of expansion at the ever increasing rate. Will it eventually attain a maximum rate of expansion? No one knows.

Dark matter and dark energy aren't the only dark things we're currently studying. There's also dark flow.

From Wikipedia:

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Dark flow is an astrophysical term describing a possible non-random component of the peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble's Law plus a possible small and unexplained (or dark) velocity flowing in a common direction.

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According to standard cosmological models, the motion of galaxy clusters with respect to the cosmic microwave background should be randomly distributed in all directions. However, analyzing the three-year Wilkinson Microwave Anisotropy Probe (WMAP) data using the kinematic Sunyaev-Zel'dovich effect, astronomers Alexander Kashlinsky, F. Atrio-Barandela, D. Kocevski and H. Ebeling found evidence of a "surprisingly coherent" 600–1000 km/s flow of clusters toward a 20-degree patch of sky between the constellations of Centaurus and Vela.

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