Let’s continue our occasional series on the basics of cosmology with a look at dark matter, an elusive substance which enabled the formation of the first galaxies, helps govern the large-scale structure of the universe, and which very likely is passing through your body right now completely unnoticed. In this article, we look at what dark matter is not. In the next, we examine a few ideas about what dark matter might be.
First a quick review…
The first inkling of the existence of dark matter came in 1933 when the visionary Swiss-American astronomer Fritz Zwicky noticed the visible mass of the Coma galaxy cluster could not account for the speedy motion of its constituent galaxies. He suggested there must be hundreds of times more invisible matter than visible matter in the cluster. He called this invisible material dunkle Materie — German for “dark matter”– because it had mass and therefore gravity but emitted no light.
More strong evidence for dark matter came in the 1970’s from the work of Vera Rubin and Kent Ford. They measured of the so-called rotation curve of spiral galaxies, which maps out the rotational speed of stars around the galaxy versus the distance of the stars from the center of the spiral. Again, their results were strong indirect evidence of dark material far beyond the visible disk. What’s more, theoretical calculations suggest the visible disks of spiral galaxies cannot exist for long unless they’re embedded in a massive spherical cloud of material.
Yet more evidence of missing matter comes from gravitational lensing of background objects by foreground galaxy clusters. There are simply not enough visible stars or gas or dust in these galaxies to account for the amount of gravitational lensing observed.
By the mid-1980’s, when I was a graduate student, many casual observers (including me) believed the clouds of missing mass were just some type of ordinary gas or dust just beyond detection, and that it was a matter of time until astronomers found the missing matter using larger telescopes and more sensitive detectors.
But nature is stranger than that. It turns out dark matter may be something completely new and exotic. And it must exist in great quantities: astronomers suspect dark matter makes up for nearly 84% of the matter in the universe.(*)
Let’s have a look at a few early ideas about the nature of dark matter based on known material and celestial objects, and why each idea was rejected by scientists…
Gas Clouds. Hydrogen makes up about 75% of “regular” matter in the universe. So perhaps dark matter is simply huge clouds of unobserved hydrogen gas. But cold clouds of gas reveal their presence by absorbing light from background galaxies, while hot gas would emit X-rays. Both effects are observed, so there is certainly hydrogen gas between galaxies, but not enough to explain the missing mass.
Rocks and Dust. How about huge clouds of comets or rocks? Snowballs of hydrogen would quickly evaporate into gas (see above), so that won’t work. And since heavier material like carbon and silicon was made only in tiny quantities in stars, there can’t be enough to come close to produce enough rocky matter to account for the observed effects of dark matter.
Antimatter. Antimatter consists of particles of the opposite charge and spin of matter. It exists in tiny quantities in our universe, but when an anti-particle interacts with “regular” matter, energy and more particles are created. If galaxies were surrounded by clouds of antimatter, astronomers would detect the telltale sign of its interaction with regular matter. But they do not.
MACHOs. “Massive Compact Halo Objects” include faint stars, planets, white dwarf stars, neutron stars, and black holes that might collect in halos around galaxies. These objects are out there. But painstaking observations over the past 20 years suggest there’s simply not enough of them to account for the missing material between galaxies.
Neutrinos. These are speedy little particles created during nuclear reactions that move through the universe in great quantities at nearly the speed of light. Every second, billions pass through your body– and the Earth itself– without slowing down. Neutrinos were once considered as strong candidates for dark matter. Alas, recent work shows they are too light and they move far too quickly– they are too “hot”– to enable the formation of large, slow-moving galaxy clusters.
Mini Black Holes. At the center of every large galaxy there’s a supermassive black hole with a mass of millions of suns. And when they expire, large stars can collapse to form black holes of a few solar masses. There aren’t enough of these two types of black hole to account for dark matter. Bu there may a third type of black hole, so-called primordial black holes created in the flash of the Big Bang. These objects may have a mass about the same as Earth, and there may be trillions of them floating around the universe. However, by their nature they’re difficult to detect and there’s no proof they exist at all. Astronomers cannot yet count them out as candidates for dark matter, but it makes them a little uneasy to pin dark matter on a type of object that cannot be seen.
Mini black holes aside, with no other obvious candidates based on existing matter as we understand it, astrophysicists now believe dark matter is very likely an exotic and as-yet undiscovered type of particle or particles that have mass but which interact very weakly with ordinary matter, emits no detectable radiation, move slowly compared to the speed of light, and which surround galaxies and permeate galaxy clusters. They must also live a long time, since evidence suggests dark matter has been around since the Big Bang.
It’s a little unsettling that some 84% of the matter in the universe has so far remained unobserved and undetected, and very likely consists of particles that have yet to be discovered. But scientist love this sort of puzzle. The search is on for dark matter, and in the next article in this series you get a look at the leading candidates for particles that could account for this mysterious substance.
(*) We think of mass and energy as different, but when it comes to calculating the shape of evolution of the universe according to Einstein’s theory of gravitation, they are equivalent. The current view is that the mass-energy of the universe is made from 5% ordinary matter (atoms and molecules), 27% dark matter, and 68% dark energy (which is also not understood). So 84% of matter is dark matter, while 95% of the universe is made of dark matter and dark energy.