Although no one has ever seen dark matter, and no one really knows what it is, scientists agree that it represents around 27 percent of the universe. This invisible material is all around us, while the matter that forms all known substances - such as atoms and subatomic particles - stands at only five percent.
Researchers have been working to detect dark matter for decades, inventing numerous mechanisms to capture the particles that it is thought to consist of, and coming up with experiments to re-create dark matter particles - for example, by using extremely high temperatures in order to collide particles of ordinary matter. While such a collision would not permit scientists to actually see a dark matter particle, detectors would be able to trace associated energy loss that would reveal that the dark particle had indeed been created.
So far, these efforts have been unsuccessful. A recent article published by a research team from the University of Southern Denmark, however, argues that perhaps we need to look at dark matter in a new way.
The traditional theory of dark matter holds that the material is lightweight and, as a result, interacts only weakly with regular matter. Labeled as "weakly-interacting massive particles," or WIMPs, dark matter particles are commonly assumed to have been produced in massive amounts after the birth of the universe, placed at around 13.7 billion years ago.
"But since no experiments have ever seen even a trace of a WIMP, it could be that we should look for a heavier dark particle that interacts only by gravity and thus would be impossible to detect directly," said Martin Sloth, associate professor at the University of Southern Denmark's Centre for Cosmology and Particle Physics Phenomenology (CP3-Origins) and co-author of the new study.
Instead, the model that Sloth and his team proposes, called Planckian Interacting Dark Matter (PIDM), calculates how the required amount of the heavy particle would have been produced in the early universe.
"It was possible, if it was extremely hot. To be more precise[,] the temperatures in the early universe must have been the highest possible in the Big Bang theory," Sloth explained.
This condition can in fact be tested. "If the universe indeed was as hot as calculated in our model, several gravitational waves from the very early childhood of the universe would have been created. We might be able to find out in the near future," he outlined.
Sloth and his team have planned for 10 separate experiments worldwide to test their new proposal. The experiments will aim to detect signals from primordial gravitational waves. To do so, they will monitor the polarization of cosmic background radiation at ground level, or from devices set up on balloons or satellites.
"If these experiments do not detect such signals," Sloth noted, "then our model will be falsified. Thus gravitational waves can be used to test our model."
