What if we could see dark matter?

We're used to thinking about the universe in terms of what we can see. When we look at the night sky, we see stars, planets, and the Moon shining against the deep black background of space. When astronomers look at the sky with powerful telescopes, some that can detect light our eyes can't see, many more stars and galaxies show up.

All those bright objects the astronomers and we see are made of matter. We know that all matter attracts all other matter through a force called gravity. Earth's gravitational force is what keeps us on the ground, since we too are made of matter. Pole vaulters, elevators, planes and rockets have to work hard to overcome the force of gravity. The same force that holds you and me to earth's surface also keeps Earth-orbiting spacecraft and even the Moon from flying away. The Sun's gravity holds earth and all the other planets, comets, and asteroids in our solar system in their orbits around it. 

Scientists understand exactly how objects behave when pulled on by gravity. They have mathematical formulas to calculate orbits and the movement of objects that are attracting each other. This is how they can send a spacecraft to meet up with a particular comet at a particular moment in time, or predict exactly when Mars will be closest to Earth.

Now when astronomers look carefully at a galaxy, they can measure how fast the stars within it are moving. the motions of the stars are the result of the gravitational forces from all other matter in the galaxy. But here is the key problem: When astronomers add up all the matter in all the matter in all the stars and gas and dust visible with all different kinds of telescopes, it doesn't total nearly enough to explain the motions they observe. the stars are moving around much faster than they should be! in other words, all the matter we can see is not enough to produce the gravity that is pulling things around. This problem shows up over and over again almost wherever we look in the universe. Not only do stars in the galaxies move around faster than expected, but galaxies within groups of galaxies do too. In all cases, there must be something we can't see, something dark.

Primary evidence for dark matter comes from calculations showing that many galaxies would fly apart, or that they would not have formed or would not move as they do, if they did not contain a large amount of unseen matter. Other lines of evidence include observations in gravitational lensing and in the cosmic microwave background, along with astronomical observations of the observable universe's current structure, the formation and evolution of galaxies, mass location during galactic collisions, and the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass- energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of an unknown form of energy known as dark energy. Thus, dark matter constitutes 85%of total mass, while dark energy plus dark matter constitute 95% of total mass-energy content. For instance, according to standard physics, stars at the edges of a spinning, spiral galaxy should travel much slower than those near the galactic center, where a galaxy's visible matter is concentrated. but observations show that stars orbit at more or less the same speed regardless of where they are in the galactic disk. This puzzling result makes sense if one assumes that the boundary stars are feeling the gravitational effects of an unseen mass- dark matter - in a halo around the galaxy.

All the protons, neutrons and electrons that make up our bodies, our planet and all the matter we're familiar with, as well as some photons (light, radiation, etc.) thrown in there for good measure. Protons and neutrons can be broken up into even more fundamental particles- the quarks and gluons - and along with the other standard Model particles, make up all the known matter in the Universe. The big idea of dark matter is that there's something other than these known particles contributing in a significant way to the total amounts of matter in the Universe.

This is the great hope: for direct detection. Because we don't know what's beyond the standard model - we've never discovered a single particle not encompassed by it - we don't know what dark matter's particle (or particles) properties should be, should look like, or how find it. We don't even know is it's all one thing, or if it's made up of a variety of different particles.

So we look at what we'd be able to detect instead, and look there. We can look for interactions down to a certain cross-section, but no lower. We can look for energy recoils down to a certain minimum energy but no lower. And at some point, experimental limitations- natural radioactivity, cosmic neutrons, solar/cosmic neutrinos, etc.- make it impossible to extract a signal below a certain threshold.

Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light, making it extremely hard to spot. In fact, researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to outweigh visible matter roughly six to one, making up about 27% of the universe. Here's a sobering fact: the matter we know and that makes up all stars and galaxies only accounts for 5% of the content of the universe! But what is dark matter? One idea is that it could contain "super-symmetric particles" - hypothesized particles that are partners to those already known in the Standard Model. Experiments to the Large Hadron Collider (LHC) may provide more direct clues about dark matter.

Many theories say the dark matter particles would be light enough to be produced at the LHC. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum "missing" after a collision. Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as super-symmetry and extra dimensions. One theory suggests the existence of a "Hidden Valley", a parallel world made of dark matter we know. If one of these theories proved to be true, it could help scientists gain a better understanding of the composition of our universe and, in particular, how galaxies hold together.