By Paul Rincon
BBC News science reporter, in Birmingham
Astronomers are shedding light on the phenomena known as "fossil" galaxies.
Chandra sees 10-million-degree gas (blue) around NGC 6482
These are isolated giant galaxies that have grown over time - the result of mergers between galaxies in a group.
Their creation may begin in the close encounter of two large galaxies which spiral together and then pull in all the smaller satellites around them.
Fossil galaxies are now being studied using the latest generation of X-ray telescopes, according work presented at the UK National Astronomy Meeting.
Researchers say the fossils' cores are packed with dark matter, as well as some normal matter.
In fact, the concentration of mass at the centre of these systems is typically five times higher than in a normal galaxy of similar size.
Mysterious dark matter has not been detected directly, but it can be inferred because it has a gravitational influence on normal matter.
The existence of fossil galaxies was confirmed in 1994. Even so, astronomers actually know very little about them.
A group at Birmingham University, UK, has identified six likely fossil systems using the US space agency's (Nasa) Chandra X-ray Observatory and the European Space Agency's XMM-Newton space telescope.
The team was especially interested in the galaxy NGC 6482. This isolated star system is the closest known fossil galaxy - being some 100 million light-years away in the constellation Hercules - which allows it to be studied in detail.
The cores of these fossils have collapsed entirely.
"This gives us a clue that these are very old objects," co-researcher Habib Khosroshahi told the BBC News website.
This must have happened at a time when the Universe was younger and more dense, while the halo of gas and dark matter around the galaxy was acquired later on.
The team now wants to study fossil galaxies in more detail using data from Chandra and XMM Newton.
The Royal Astronomical Society's National Astronomy Meeting is being held at the University of Birmingham.