After a decades-long search, astronomers may finally have found the first sign of dark matter. That's the invisible substance that scientists believe makes up the bulk of our universe, since visible matter accounts for only about 20 percent of our universe by weight.

While scientists can observe dark matter indirectly by looking at its gravitational effects on visible matter, they have struggled to come up with tangible evidence that proves the stuff exists--until now.

This week, a team of researchers from Switzerland and the Netherlands announced that they may have detected the signal of decaying dark matter particles.

For the research, the team analyzed the x-rays emitted from two celestial objects: the Perseus galaxy cluster, an array of galaxies located approximately 250 million light years from Earth, and our "sister" galaxy Andromeda, which is approximately 2.5 million light years away. The researchers looked at data collected by the European Space Agency's XMM-Newton telescope and spotted a mysterious "anomaly" that could not have been emitted by any known atom or particle.

The same strange x-ray spike was also detected by a research team at Harvard in June, who announced they had spotted the emission in data from 70 different galaxy clusters.

"This tiny (several hundred extra photons) excess has been interpreted as originating from very rare decays of dark matter particles," Dr. Alexey Boyarsky, a professor of physics at Leiden University in the Netherlands and the lead researcher for the new study, told The Huffington Post in an email. "Although the signal is very weak, it has passed several 'sanity checks' that one expects from a decaying dark matter signal."

For instance, the researchers say the signal was more concentrated in the center and weak at the edges of Andromeda and the Perseus cluster, which corresponds to what they expected. Boyarsky added that the team has now found a signal at the same wave length coming from our own galaxy, the Milky Way.

Boyarsky and his team believe the signal comes from the decay of a dark matter particle, possibly a "sterile neutrino," which is a hypothetical particle believed to be 1/100th the size of an electron.

"Confirmation of this discovery may lead to construction of new telescopes specially designed for studying the signals from dark matter particles," Boyarsky said in a written statement. "We will know where to look in order to trace dark structures in space and will be able to reconstruct how the Universe has formed."