Only 5% of the universe is visible to us
Investigating the composition of our Universe we find that only 5% of it consists of matter that we know; matter that consists of electrons, protons, and neutrons and emits and absorbs photons and neutrinos. We find that there are two more components to the energy content of this Universe: Dark matter and dark energy. These two components at this current stage of the evolution of our Universe constitute 27% and 68% of its total energy content. We call these components dark since unlike normal matter they do not emit or absorb any form of light: we can not “see” them, not even indirectly by them “blocking” light from other objects like stars or galaxies.
There is no doubt dark matter exists
The presence of dark matter manifests itself in many astronomical observations as its gravitational pull alters the motion of astronomical objects. In the late 1970-ties its gravitational effect was found in the rotation of the stars in galaxies. These days we use dark matter's gravitational effect on light itself as for example it passes by a galaxy cluster to measure the distribution of dark matter within the galaxy cluster and its galaxies.
Dark matter gave the Universe its form
When we look through our telescopes we see that galaxies are huddled together in galaxy clusters and galaxy clusters form a three dimensional web permeating the universe with vast voids between the filaments of this web. These filaments and voids originate from small density fluctuations in the “matter soup” as our Universe emerged from the Big Bang. At that time dark energy did not yet play a role in shaping the Universe. And with five times more dark than normal matter in the mix, the evolution and the gravitational pull of dark matter were what shaped this web of matter that we see when we look through our telescopes.
Through our high precision measurements of the cosmic microwave background, an “echo” of the Big Bang, and of the current structure and dynamics of objects in the Universe we live in, we can learn a lot about the nature of dark matter. The main characteristics a dark matter particle must have are:
• it must be either stable or at least live longer than the Universe exists
• it must be cold: either heavy or slow enough to be trapped in its own gravitational potential
• it can not have any interaction with normal matter that is stronger than at most the weak interaction
There is no known particle that has all these characteristics. We do not know what dark matter is, only that it exists and shaped the universe we live in.
Dark matter: Gateway to new physics
From the movement of stars in our Milky Way galaxy we can estimate the density of dark matter in the vicinity of our Sun: dark matter equivalent to 1/3 of the mass of a proton must be contained in one cubic centimeter of space here on Earth. As none of the particles we know about can be the dark matter particle, we do not know in what way – if any – other than through gravity a dark matter particle might interact with normal matter. In this situation all we can do is build the most sensitive detector we can and hope that this enables us to see the interaction of a dark matter particle some day.
If this can be achieved, it would be a scientific breakthrough of tremendous importance. We would learn which theory to choose to extend our knowledge of normal matter to this mysterious dark sector of the universe. And that in turn would likely teach us knew things about the matter and particles that we already know.
Dark matter candidates beyond the physics of known particles:
Our standard model of particle physics explains everything we know about normal matter. Yet it seems incomplete: it has some features that would be easy to understand if there were other particles beyond its borders. Yet we never found any of these. Dark matter is the only hint that there indeed should be something else out there. But which theory about this physics beyond that we should trust?
Among the many extensions of known particle physics that offer a dark matter candidate particle, two are of particular interest to experimentalists: The WIMP and the Axion. They each solve a different perceived problem with the theory that we know. And they both would interact with normal matter, giving us a chance to detect them. Unlike the other LXe detectors XMASS is optimized to find the smallest possible signals more than to hunt for one specific choice of theory that could provide a dark matter candidate. XMASS can look for both, WIMPs and Axions, as well as many other candidates. This is reflected in the results obtained with the current XMASS detector.