Strands in the Cosmic Web

The Cosmic Web

Throughout much of the 20th century, it was an open question in astronomy as to what the universe looked like on the largest observable scales. Were galaxies and galaxy clusters distributed uniformly throughout space, or was there a pattern? Thanks to galaxy surveys like the CfA Redshift Survey and more recently the Sloan Digital Sky Survey we know that on large scales, the matter distribution of the universe is clumpy instead of smooth. Through these surveys we observe directly the distribution of luminous matter like stars, gas, and galaxies. However, luminous matter comprises only a small fraction of the matter in the universe (17%), the rest is dark matter which interacts via gravity but does not absorb and emit electromagnetic radiation like normal matter (at least not as far as we know, though several claims have challenged that notion, see Nathan and Elizabeth’s recent astrobites here and here). Theoretical simulations of dark matter cosmologies like the Millennium Simulation (Fig 1) predict that there is a dark matter backbone to the cosmic web, with filaments of dark matter stretching between clusters of galaxies. To observe this distribution of dark matter, however, we must resort to more indirect methods than those used to detect the stars and galaxies. Although the existence of dark matter filaments is a firm prediction of standard cosmological models there has not yet been a robust detection, until now.

Figure 1: The dark matter distribution of the universe on extremely large scales. At the intersection of the filaments are the largest luminous structures in the universe: galaxy clusters. From Springel et al, Millennium Simulation.

Measuring Dark Matter

The evidence for dark matter has been established on many size scales of the universe, from the unexpected rotation velocities of galaxies to the determination of the cosmological parameters from the Cosmic Microwave Background. The presence of dark matter in galaxy clusters can be inferred by the lensing effect that mass has on passing light. Strong gravitational lensing occurs when the gravitational lens creates an image of the background source, while weak gravitational lensing distorts the shapes of background sources, stretching or compressing them in certain directions (called shear). For more about the weak lensing effect, see Elizabeth’s astrobite.

Observing A Dark Matter Filament

At the nodes of the cosmic web there lie the largest collections of luminous and dark matter. We see the luminous matter in the form of galaxy clusters, massive collections of hundreds or thousands of galaxies, while we infer the dark matter from dynamical and gravitational lensing arguments. We can see the strands, or filaments, of the cosmic web connecting these nodes via the galaxies that live there, but until now there has been no robust determination of the bridge of dark matter that should also exist. Dietrich et al. reconstruct the matter distribution of the Abell 222 and Abell 223 supercluster system (at redshift z=0.21) using weak gravitational lensing to infer the existence of a dark matter filament at a 4.1 sigma significance (for a thorough discussion of sigmas and confidence levels in the context of the Higgs Boson, see Shannon’s recent astrobite).

In Figure 2, the authors show a visible light image of the supercluster system with the surface density of the matter (both luminous and dark) overplotted in blue with contours. Such a unique discovery naturally raises questions about its plausibility.

  1. First, does the bridge actually contain dark matter, or can its mass be explained by luminous matter only?
  2. Second, is the bridge actually a filament between clusters, and not just the tips of two elliptically shaped clusters overlapping?
  3. And lastly, theory predicts that dark matter filaments should be extremely difficult to observe via lensing, why were the authors able to detect this one?

Figure 2: The Abell 222 and Abell 223 galaxy clusters. The surface density of the matter distribution (both luminous and dark) in shown in blue with contour levels. A filament of dark matter connects the two galaxy clusters.

That the bridge of matter connecting the two clusters is a dark matter filament is supported by many of the authors’ arguments.

#1: The authors tackle this question by observing the filament in X-rays. As explained by Dan, galaxy clusters emit very strongly in the X-ray, and the X-ray luminosity is proportional to the square of the density of the medium. By assuming the distribution of the hot gas in the filament is uniform, the authors determine an upper limit on the fraction of the filament mass that is hot gas. If the same amount of gas were clumpy, the X-ray emission would be larger because denser regions would disproportionately emit more X-rays. Thus, the authors account for all luminous matter in the filament. Any extra mass inferred from gravitational lensing must be of the dark variety.

#2: To show that the dark matter between the clusters is actually a filament and not part of the two clusters, the authors fit the galaxy clusters with elliptical profiles and the filament with a simple straight bar of mass and use a Markov Chain Monte Carlo algorithm to determine the best fit parameters. Essentially, they find that regardless of ellipticity or rotation angle of the galaxy clusters, a model with a filament connecting the clusters is robustly preferred over a model with no filament for a wide range of parameters.

#3: Theoretical predictions that claim filaments should be unobservable assume that the filaments are oriented in the plane of the sky, such that they would have the maximum extent on the projected surface of the sky, but have little depth (let’s call this configuration #1). If instead the filament was aligned in a perpendicular way, such that it has a minimum extent on the projected surface of the sky but a maximal depth (configuration #2), the gravitational lensing signature would be stronger. To explain why this orientation of the filament is not so unlikely, the authors invoke a cosmological argument based on the timing of the formation of the clusters. If the redshift difference of the galaxy clusters is due solely to the expansion of the universe, then the line-of-sight separation is 18 megaparsecs. To otherwise explain the redshift difference due the peculiar motions of the galaxy clusters would require a combined cluster mass more than 10 sigma of what the authors estimate. The combination of these arguments means that it is likely that we actually are seeing the filament lengthwise (configuration #2), giving the maximal gravitational lensing signature.

The first robust detection of a dark matter filament is exciting news, and even further proof of the cosmic web of our universe. Future missions like WFIRST and Euclid will hopefully bring many more insights into the large scale structure of our galaxy.

About Ian Czekala

I am a second year graduate student at the Harvard-Smithsonian Center for Astrophysics. I work with Edo Berger on studying intermediate luminosity optical transients discovered with Pan-STARRS.

Discover more from astrobites

Subscribe to get the latest posts to your email.


  1. Thanks for this helpful synopsis, Ian. I have read Nature’s synopsis (see here: and others, and most seem to claim that this result is a “direct detection” of dark matter. How is this possible? I thought that (independently of how “robust” their methodology is) direct detection could only be achieved via a dark matter signal from essentially the creation of a dark matter particle (whichever of the many candidates it might be). Can you or anyone else clarify this?

  2. Hi Al,

    Thank you for your comment. I think what might be going on is that certain authors have a different interpretation of what “direct” might mean in different contexts. Certainly a signal from the annihilation of dark matter either in the center of Milky Way or found via the Large Hadron Collider would be very direct (and in the case of the LHC, subject to reproduction)! In the case of this discovery, however, I believe the excitement is over the “direct detection” of a dark matter filament, such that we see the lensing signature from a particular filament, as opposed to inferring the presence of filaments from the distribution of luminous matter like galaxies.

    Thank you,


  3. Hi,
    I tried to examine your results.
    Please help me to answer two questions:
    How did you get data for the X-ray luminosity (source) in the filament?
    I get a source LP 708-182 = NLTT 5447 within the filament moving to ACO 222. How would you interpret this? Can there be a context with your results? Can dark matter maybe accelerate this process?
    Regards from Germanby
    Klaus Lang

  4. Hi Klaus,

    Your question might be answered better by the authors of the paper than by me. Their contact info is available through the posting on the arXiv itself, available here:

    Thank you,


  5. @Klaus Lang

    Details of x-ray observations of the filament by XMM-Newton can be found in the 2008 paper by Werner et al:

  6. The filaments in the Cosmic Web , in my opinion, are a result of the overlapping of Baryonic Acoustic Oscillations fromn the hotspots of the CMBR image after the era of recombination.



  1. Into the Void | astrobites - [...] building the complicated structure of galaxies and clusters we see today, which we call the cosmic web. You’ve probably…
  2. A deep X-ray observation of Hickson Compact Group 62 | astrobites - [...] They also contain dark matter (which was probably instrumental in the cluster formation: see Ian’s astrobite) and a massive…

Leave a Reply