An orbital X-ray telescope has found a chunk of matter whose existence had long been theorized, but evidence for which had been lacking, the European Space Agency (ESA) said on Wednesday.

The discovery made by ESA's XMM-Newton telescope is part of so-called baryonic matter, which comprises less than five percent of the cosmos.

Most of the universe consists of matter and energy of an unknown nature, which astrophysicists call "dark matter." It is believed to be distributed in a web-like structure.

The new claim is based on the observation of a pair of distant galaxy clusters, called Abell 222 and Abell 223, located 2.3 billion light-years from Earth.