The distances to the galaxies that they are in show that the universe appears to have been expanding more slowly in the past than now. So not only is everything rushing away from everything else, but, these days, it’s also rushing away more quickly. In other words, the expansion of the universe is accelerating. And what is causing it to accelerate? Well, it might be the “vacuum energy” that Einstein thought he needed to support the universe from collapsing on itself.
Interestingly, this force increases with distance—as opposed to gravity, which weakens with distance. Scientists have dubbed this new energy required to power an accelerating expansion “dark energy,” implying that, as with “dark matter,” we know it is there, but we don’t know what it is. The expansion of the universe appears to be accelerating, powered by an as yet unexplained “dark energy.” The universe consists of 4 percent “normal” atoms (the stuff we are made of), 23 percent dark matter, and 73 percent dark energy.Beyond galaxy,So what is all this other stuff we cannot see directly? Whatever its makeup, it apparently emits no radiation of any kind—no visible light, no x-rays, no gamma radiation. But it cannot hide completely. We “see” it because it has mass and its mass affects the way in which the stars and gas of the Milky Way orbit. Astronomers call the region containing this mass the “dark halo,” and the Milky Way is not alone in possessing such a region. Many, if not all, galaxies have the same signature in the rotation of their stars and gas. Presumably, the dark halo contains—what else?—dark matter, a catchall term that we use to describe a variety of candidate objects. The truth is, we’re not sure what dark matter is, but we do know it’s there because we clearly can see its gravitational effects.it is difficult-to-detect low-mass stars (brown dwarfs or faint red dwarfs) might be responsible for the mass in this region. And neutrinos do have nonzero mass, so their presence might contribute to dark matter.Because dark matter accounts for a great deal of the mass of a galaxy—up to 10 times more than the mass of visible matter—the shocking conclusion must be that 90 percent of the universe is dark matter, utterly invisible in the most profound sense of the word. Dark matter neither produces nor reflects any electromagnetic radiation of any sort at any wavelength. The majority of the mass (90 percent) of most galaxies and clusters (99 percent) is made of dark matter, material we cannot observe but only see by its gravitational effect. Although several lines of evidence exist that confirm dark matter is very real, we are still unsure of what it might be.