Dark matter

Dark matter is a kind of matter that is invisible to telescopes, which does not emit electromagnetic radiation and whose (presumed) existence can only be indirectly detected today through its gravitational effects. Although hypothesised for the first time as early as 1933, by the Austrian astronomer Fritz Zwicky, dark matter seriously entered the scientific debate only starting in the 1970s. This was thanks to observations of the Andromeda galaxy collected by the American astronomers Vera Rubin and Kent Ford. The two scientists measured the rotation speed of stars at the edge of the galaxy, finding a surprisingly high value compared to the predictions of Newton’s gravity. The most plausible explanation of the anomaly was admitting the existence of a kind of matter that was not visible, called “dark”, whose contribution (added to that of ordinary matter) would explain the speed measured.

Simulation of the formation of dark matter structures from the early universe to the present. (©SLAC National Accelerator Laboratory, Ralf Kaehler)

Representation of the rotation curve of a typical spiral galaxy. The blue dashed curve is the prediction of the Newtonian law of gravity; the green one represents the trend of the experimental measurements (©INFN)

After the discovery by Rubin and Ford, the hypothesis of dark matter became increasingly popular in the scientific community, including thanks to other indirect evidence that goes beyond the mere problem of the rotation speed of galaxies. Today, the expected abundance of dark matter is precisely known: it is equal to approximately 85% of the overall mass of the universe and 27% of the mass-energy balance.
Over the last few decades, theoretical physicists have proposed a large number of models that attempt to explain dark matter. One of the most popular predicts the existence of the so-called WIMP (weakly interacting massive particles). These hypothetical particles are relatively heavy and weakly interacting and, for several reasons, would have almost perfect features for constituting dark matter.

At the same time, numerous, increasingly complex and sensitive experiments dedicated to researching WIMP and other dark matter candidates were undertaken. Some of the most important internationally, like DAMA and XENON, are hosted at the INFN Gran Sasso National Laboratories.
Until now, no evidence of the observation of WIMP, or other potential dark matter particles, has been gathered. While the community intends to continue experimental efforts with even more sensitive experiments, researchers are starting to more seriously consider alternative hypotheses to dark matter as well.

Xenon experiment at the Gran Sasso National Laboratories (© INFN)

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