What's the Matter? - Unsolved Mysteries Week


Intergalactic dark matter pipeline between two galaxies. Photo courtesy of Wikimedia Commons.

There’s something out there that we can’t see. Stretching out in all directions and filling the space between all known matter in our infinite universe, lurks an invisible and mysterious element. This evasive particle is known only as Dark Matter and it constitutes over 90 percent of our universe. The question of dark matter is currently the most profound mystery in cosmology and astrophysics and is largely considered one of the most outstanding problems in theoretical physics. It is called ‘dark matter’ because it neither emits nor reflects any light, but is known only due to the strong gravitational pull it exerts on visible matter. Just 15 years ago its existence wasn’t known at all. Since the discovery of its presence, physicists have been scratching their heads and applying numerous theories to try and figure out just what this unknown element is. But although theories abound, the true nature of this enigmatic element has yet to reveal itself.

In 1998 a discovery was made that shocked and galvanized the cosmology community, the universe was not remaining stable or slowing down – it was expanding, and speeding up. This was something that, according to all sound theoretical forecasts, should not be happening. Given all of the gravitational force that was being exerted by all the matter of space, the universe should be experiencing an attractive inward pull that would slow its expansion. But in 1998, two teams of astronomers announced the overlapping results: Studies of distant supernovas (exploding stars) showed that stars looked dimmer than expected. The only way to account for this was if the stars were somehow further away than expected, but the physicists had already calculated and accounted for the specific amount of expansion of the universe. The explanation they eventually found was startling: the rate of expansion of the universe was accelerating.

In August 2007 scientists from a NASA program stumbled upon an enormous hole in the universe– one spanning one billion light years and located about six to 10 billion light years away from Earth. This “hole” contained no stars, no planets, no solar systems or even clouds of cosmic dust. Though this “hole” appears empty, it is believed to be filled with dark matter. The program that was being explored at the time of this discovery was one dedicated to exploration of the spread of radiation emitted from the Big Bang, a significant fact, since one hypothesis is that dark matter is actually a reverberating remnant of the Big Bang.

Essentially, the gravitational effects observed in our universe don’t match the amount of matter seen, so there has to be an element at play that is somehow counterbalancing or even over-riding the effects of gravity, which should be pulling everything closer together and thus slowing the expansion of the universe- but is not. Physicists decided this had to be some sort of “repulsive gravity“, but unfortunately this is about as far as they have gotten with figuring out dark matter’s essential structure. Among the theories postulated is the idea that dark matter is some sort of “ordinary” (previously known) matter such as brown dwarfs (weak stars) or black holes (remnants of dead stars). But both of these phenomena are broadly considered too rare to account for the enormous energy that dark matter emits. The other possibility is that dark matter is “extraordinary matter”, an entirely new type of matter that has never been seen or understood before. Examples of this (theoretically) are subatomic particles that interact weakly with ordinary matter known as WIMPs (Weakly Interacting Massive Particles). The leading extraordinary matter theory is “neutralinos”- massive subatomic particles that move near the speed of light. Any extraordinary matter theory will take considerable time, energy, and study to figure out since there is little precedent to build upon.

“Understanding dark matter would help us understand the large scale structure of the Universe, why it formed the way it did, and how that might change in the future. This is a pretty big question. As you can imagine, discovering the nature of 80 percent of the matter in the Universe will have an incredible effect of how we understand it’s past, present, and future. It’s an odd thing to think about. We do not understand what most of the Universe is made of,” says Tom Schwarz, an experimental particle physicist from the University of Michigan.

“Uncovering the truth of dark matter would require us to accept a new particle (and likely new forces) beyond the standard model of subatomic particles and their interactions. I would call its discovery very disruptive in that regard,” he says. When it comes to the dark matter mystery, progress is slow, and it is very possible that the most exciting questions have yet to be asked.

Many physicists are reluctant to accept an explanation for dark matter that does not fit into the “unquestionable” framework of the Big Bang theory, but the amazing thing about dark matter is that some of the most unconventional and intriguing theories are amongst the most likely. One such breathtaking theory is that the repulsive gravity of dark matter is caused by the existence of unseen parallel universes. In May of 2011 NASA sent a particle physics detector deemed the Alpha Magnetic Spectrometer into space to explore this very possibility.

“What we are confirming is that the Universe is filled with stuff that has little to do with the stuff that makes us up. It’s funny how scientific progress always seems to make humans feel less important,” says Schwarz. “Originally, we thought we were the center of the solar system. Later on, we thought we were the only galaxy, And now, we are just an afterthought in the makeup of the Universe.” Now these very “afterthoughts” are fumbling in obscurity, left to ponder- what cosmic ingredient is lurking in the darkness? Undetectable, constant, elusive, and ancient…