The gravitational lensing map (blue), overlayed over the optical and X-ray (pink) data of the Bullet cluster. The mismatch of the locations of the X-rays and the inferred mass is undeniable. (X-RAY: NASA/CXC/CFA/M.MARKEVITCH ET AL.; LENSING MAP: NASA/STSCI; ESO WFI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.; OPTICAL: NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

The Most Important X-Ray Image Ever Taken Proved The Existence Of Dark Matter

NASA’s Chandra X-ray observatory has shown us the Universe like nothing else ever before.


When it comes to the Universe, it mainly generates X-rays through high-temperature heating.

X-rays from Chandra reveal the cluster MACS J0717’s hot gas, while optical data show the individual galaxies in the system. (X-RAY (NASA/CXC/IFA/C. MA ET AL.); OPTICAL (NASA/STSCI/IFA/C. MA ET AL.)

When matter heats up, through collisions, interactions, acceleration or collapse, it can emit X-rays.

X-ray emissions that are large, extended, and structure-rich highlight a variety of supernovae seen in the galaxy. Some of these are only a few hundred years old; others are many thousands. A complete absence of X-rays indicates the lack of a supernova. In the early Universe, this was the most common death-mechanism of the first stars. (NASA/CXC/SAO)

Galaxy clusters, supernova remnants, active galaxies, binary star systems, and even the Moon emit them.

As seen in X-rays against the cosmic background, the Moon’s illuminated (bright) and non-illuminated portions (dark) are clearly visible in this early X-ray image taken by ROSAT. The X-rays arise mostly from reflected emission from the Sun. (DARA, ESA, MPE, NASA, J.H.M.M. SCHMITT)

Yet the most important X-ray image of all time was an incredible surprise.

The Bullet cluster, the first classic example of two colliding galaxy clusters where the key effect was observed. In the optical, the presence of two nearby clusters (left and right) can be clearly discerned. (NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

This is the Bullet Cluster: a system of two galaxy clusters colliding at high speeds.

The X-ray observations of the Bullet Cluster, as taken by the Chandra X-ray observatory. (NASA/CXC/CFA/M.MARKEVITCH ET AL., FROM MAXIM MARKEVITCH (SAO))

As the gaseous matter inside collides, it slows, heats up, and lags behind, emitting X-rays.

Optical images from the Magellan telescope with overplotted contours of the spatial distribution of mass (left), from gravitational lensing. When you look at those same contours overplotted over Chandra X-ray data that traces hot plasma in a galaxy (right), you can see that the normal matter and the overall effects of mass do not align. (D. CLOWE, M BRADAČ, A. H. GONZALEZ ET AL., APJ (2006))

However, we can use gravitational lensing to learn where the mass is located in this system.

The bending and shearing of light from background galaxies shows it’s separated from the matter’s and X-rays’ location.

Large-field mass reconstruction based on the combined (HST and CFHT) catalogs. On the left-hand side, the mass contours of Abell 520 are overlaid on the smoothed rest-frame luminosity distribution of the cluster. On the right-hand side, the distribution of the high (red) and low (green) velocity groups, corresponding to the multiple mass centers of the cluster. (M.J. JEE ET AL. (2012), THE ASTROPHYSICAL JOURNAL, VOLUME 747, NUMBER 2)

This separation is some of our strongest evidence for dark matter.

Three colliding galaxy clusters (and one colliding group, at the lower-left), showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. On large scales, cold dark matter is necessary, and no alternative or substitute will do. (X-RAY: NASA/CXC/UVIC./A.MAHDAVI ET AL. OPTICAL/LENSING: CFHT/UVIC./A. MAHDAVI ET AL. (TOP LEFT); X-RAY: NASA/CXC/UCDAVIS/W.DAWSON ET AL.; OPTICAL: NASA/ STSCI/UCDAVIS/ W.DAWSON ET AL. (TOP RIGHT); ESA/XMM-NEWTON/F. GASTALDELLO (INAF/ IASF, MILANO, ITALY)/CFHTLS (BOTTOM LEFT); X-RAY: NASA, ESA, CXC, M. BRADAC (UNIVERSITY OF CALIFORNIA, SANTA BARBARA), AND S. ALLEN (STANFORD UNIVERSITY) (BOTTOM RIGHT))

Since then, over a dozen additional colliding clusters display such a separation, in a variety of configurations.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. Alternative theories now need to be so contrived that they are considered by many to be quite ridiculous. (X-RAY: NASA/CXC/ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND/D.HARVEY NASA/CXC/DURHAM UNIV/R.MASSEY; OPTICAL/LENSING MAP: NASA, ESA, D. HARVEY (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND) AND R. MASSEY (DURHAM UNIVERSITY, UK))

Whatever dark matter is, it cannot be accounted for by the Universe’s normal matter alone.

The large-scale clustering data (dots) and the prediction of a Universe with 85% dark matter and 15% normal matter (solid line) match up incredibly well. The lack of a cutoff indicates the temperature (and coldness) of dark matter; the magnitude of the wiggles indicates the ratio of normal matter to dark matter; the fact that the curve is largely smooth and doesn’t have spontaneous drops down to zero amplitude rules out a normal-matter-only Universe. (L. ANDERSON ET AL. (2012), FOR THE SLOAN DIGITAL SKY SURVEY)

The Bullet Cluster images were the first to demonstrate this effect.

The colliding galaxy cluster “El Gordo,” the largest one known in the observable Universe, showing the same evidence of dark matter as other colliding clusters. It is possible to explain El Gordo with new physics, but this is an unnecessary complication; standard collisionless dark matter does just fine here, as it does for all of the colliding clusters. (NASA, ESA, J. JEE (UNIV. OF CALIFORNIA, DAVIS), J. HUGHES (RUTGERS UNIV.), F. MENANTEAU (RUTGERS UNIV. & UNIV. OF ILLINOIS, URBANA-CHAMPAIGN), C. SIFON (LEIDEN OBS.), R. MANDELBUM (CARNEGIE MELLON UNIV.), L. BARRIENTOS (UNIV. CATOLICA DE CHILE), AND K. NG (UNIV. OF CALIFORNIA, DAVIS))

NASA’s Chandra, which took the image, has been rightfully renewed as NASA’s flagship X-ray observatory after 19 continuous years.

Artist illustration of the Chandra X-ray Observatory. Chandra is the most sensitive X-ray telescope ever built, and has just been extended through at least 2024 as the flagship X-ray observatory in the NASA arsenal. (NASA/CXC/NGST TEAM)

Mostly Mute Monday tells the scientific story of an astronomical object, image, or phenomenon in visuals and no more than 200 words. Talk less; smile more.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.