Using observations from NASA’s Hubble Space Telescope and Chandra
X-ray Observatory, astronomers have found that dark matter does not slow
down when colliding with itself, meaning it interacts with itself less
than previously thought. Researchers say this finding narrows down the
options for what this mysterious substance might be.
Dark matter is an invisible matter that makes up most of the mass of
the universe. Because dark matter does not reflect, absorb or emit
light, it can only be traced indirectly by, such as by measuring how it
warps space through gravitational lensing, during which the light from a
distant source is magnified and distorted by the gravity of dark
To learn more about dark matter and test such theories, researchers
study it in a way similar to experiments on visible matter -- by
watching what happens when it bumps into other objects. In this case,
the colliding objects under observation are galaxy clusters.
Researchers used Hubble and Chandra to observe these space
collisions. Specifically, Hubble was used to map the distribution of
stars and dark matter after a collision, which was traced through its
gravitational lensing effect on background light. Chandra was used to
detect the X-ray emission from colliding gas clouds. The results are
published in the March 27 edition of the journal Science.
"Dark matter is an enigma we have long sought to unravel,” said John
Grunsfeld, assistant administrator of NASA’s Science Mission Directorate
in Washington. "With the combined capabilities of these great
observatories, both in extended mission, we are ever closer to
understanding this cosmic phenomenon.”
Galaxy clusters are made of three main ingredients: galaxies, gas
clouds, and dark matter. During collisions, the gas clouds surrounding
galaxies crash into each other and slow down or stop. The galaxies are
much less affected by the drag from the gas and, because of the huge
gaps between the stars within them, do not slow each other down.
"We know how gas and stars react to these cosmic crashes and where
they emerge from the wreckage. Comparing how dark matter behaves can
help us to narrow down what it actually is," said the study’s lead
author David Harvey of the École Polytechnique Fédérale de Lausanne
(EPFL) in Switzerland.
Harvey and his team studied 72 large cluster collisions. The
collisions happened at different times and were viewed from different
angles -- some from the side, and others head-on.
The team found that, like the galaxies, the dark matter continued
straight through the violent collisions without slowing down much. This
means dark matter does not interact with visible particles and flies by
other dark matter with much less interaction than previously thought.
Had the dark matter dragged against other dark matter, the distribution
of galaxies would have shifted.
"A previous study had seen similar behavior in the Bullet Cluster,"
said team member Richard Massey of Durham University in the United
Kingdom. "But it's difficult to interpret what you're seeing if you have
just one example. Each collision takes hundreds of millions of years,
so in a human lifetime we only get to see one freeze-frame from a single
camera angle. Now that we have studied so many more collisions, we can
start to piece together the full movie and better understand what is
With this discovery, the team has successfully narrowed down the
properties of dark matter. Particle physics theorists now have a smaller
set of unknowns to work around when building their models.
"It is unclear how much we expect dark matter to interact with itself
because dark matter already is going against everything we know,” said
Harvey. "We know from previous observations that it must interact with
itself reasonably weakly.”
Dark matter may have rich and complex properties, and there are still
several other types of interactions to study. These latest results rule
out interactions that create a strong frictional force, causing dark
matter to slow down during collisions.
The team also will study other possible interactions, such as dark
matter particles bouncing off each other like billiard balls and causing
dark matter particles to be ejected from the clouds by collisions or
for dark matter blobs to change shape. The team also is looking to study
collisions involving individual galaxies, which are much more common.
"There are still several viable candidates for dark matter, so the
game is not over. But we are getting nearer to an answer," said Harvey.
"These astronomically large particle colliders are finally letting us
glimpse the dark world all around us, but just out of reach."
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore conducts Hubble science
operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington.
NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.