Dark matter: unless you're a theoretical physicist, you probably will have a hard time explaining just what this mysterious thing in our Universe really is. Even experts have long viewed the material as one giant question mark looming over blackboards and telescopic lenses. Now, a new discovery has made things even more complicated. Dark matter, as it turns out, barely even interacts with itself, explaining why it is so undetectable.
Traditionally, scientists stand by the adage that "seeing is believing." Even if you can't see some matter with the naked eye, various instruments should be able to pick up whatever light it reflects, absorbs, or emits. However, dark matter does none of those things - a unique characteristic that earned it the name "dark" in the first place.
Still, physicists had assumed that this matter at least interacted with itself. And, supposedly, it indeed does, but to a far lesser degree than was expected.
That's at least according to a study (PDF) recently published in the journal Science, which details how dark materials don't even slow down after slamming into others of their kind during galactic collisions.
Contemporary astronomical knowledge - the Universe as we know it - stipulates that clusters are made of three main ingredients: galaxies, gas clouds, and dark matter. It is assumed that dark matter is evenly distributed throughout these clusters, so if two were to collide, you could reason that various dark material would also meet head-on.
"We know how gas and stars react to these cosmic crashes and where they emerge from the wreckage," study author David Harvey, of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, said in a recent statement. "Comparing how dark matter behaves can help us to narrow down what it actually is."
Specifically, Harvey and his team studied 72 large cluster collisions in all, using detailed imagery from NASA/ESA's Hubble Space Telescope and the Chandra X-ray Observatory to compare results. And what they found might seem a little disappointing. (Scroll to read on...)
According to observations made by Harvey and his colleagues, dark matter passed right through the collision without any notable slowing, indicating that not only does it not interact with visible particles, it also breezes by other dark matter.
But wait... if dark matter can't even influence itself, how the heck do scientists know it's there at all?
If physicists were to be completely honest, they'd probably tell you that dark matter's existence actually remains uncertain. However, this is not all that different from asking a child to explain how they know the wind is there. While you can't physically see the wind, you can see how it affects the things around you - rustling leaves, carrying loose litter, and playing with your hair.
In the case of dark matter, scientists believe it's there because they see something influencing the Universe in inexplicable ways - such as warping space through gravitational lensing. This phenomenon, which you can read about here, would not be possible if a cluster's immense gravitational pull came from galaxies and gas alone. A third element, dark matter, must also then be present to explain for its force.
"Dark matter is an enigma we have long sought to unravel," said John Grunsfeld, assistant administrator of NASA's Science Mission Directorate in Washington. "With the combined capabilities of [Hubble and Chandra] we are ever closer to understanding this cosmic phenomenon."
Harvey added that now they know how weakly dark matter interacts with itself, experts can narrow down what the invisible matter actually is.
"There are still several viable candidates for dark matter, so the game is not over. But we are getting nearer to an answer," he explained. "These astronomically large particle colliders (cluster collisions) are finally letting us glimpse the dark world all around us, but just out of reach."
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