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Tonight, fortyozspartan got me thinking about the 96% of the universe that isn't like us. The overall breakdown is 4% normal matter, 22% dark matter, and 74% dark energy. The latter is basically a force that accelerates the expansion of the universe, a sort of antigravity if you will, unrelated to dark matter except that we know next to nothing about both right now, hence the "dark," as in "the Dark Ages." Here's how NASA shows this in chart form:
But even though dark matter is invisible, we can measure its effects. Scientists measured the dark-matter-induced gravitational lensing (as described on a recent Astro-Image of the Day) shown in Hubble images and ran a computer simulation to create these dark matter maps. The maps suggest that galaxies form at the densest regions of dark matter. Considering that 90% of the universe is dark matter, it follows that galaxies and other normal-matter formations (yellow in the images) would be most dense where dark matter (blue) is most dense, forming haloes around the galaxies.
This supports the theory that dark matter and normal matter gather at the same places because of gravity, and that dark matter attracts visible matter. Interestingly, dark matter then helps form stars, galaxies, and galactic clusters. This final image is one that Myungkook James Jee's team released this year, an update of the first one above; again, blue represents dark matter while yellow and red is our kind of star-stuff:
This also supports the theory that dark-matter particles are collision-less. That is, unlike normal matter particles, dark matter does not collide and scatter but simply passes through other dark-matter particles. "Collision-less particles do not bombard one another, the way two hydrogen atoms do," says Myungkook James Jee. "If dark matter particles were collisional, we would observe a much smoother distribution of dark matter, without any small-scale clumpy structures."
The universe is truly an awe-inspiring place.