These days we know that less than a quarter of the matter in the universe is visible. The rest of the matter is what we call "dark matter". A mysterious type of matter that was formed early after the big bang, and it explains why our visible universe expands into the form of a cosmic web. The voids that are observed within this web are actually anything but voids - they are filled with dark matter. Being dark, dark matter could not be directly observed, but the gravitational effects betrayed its presence. This effect is called "gravitational lensing" and is the massive effect of the gravity of the dark matter on the light coming from objects behind it.
Dark matter was studied for the first time in 2012 using the Hubble space telescope by a team of astronomers. It was a giant filament of dark matter, extending for 60 million light-years. The filament is part of the cosmic web that constitutes the large-scale structure of the Universe, a leftover of the very first moments after the Big Bang, and it was reconstructed in 3D!
The team combined high resolution images of the region around the massive galaxy cluster MACS J0717.5+3745 (or MACS J0717 for short), taken using Hubble, NAOJ's Subaru Telescope and the Canada-France-Hawaii Telescope, with spectroscopic data on the galaxies within it from the WM Keck Observatory and the Gemini Observatory. Analysing these observations together gives a complete view of the shape of the filament as it extends out from the galaxy cluster almost along our line of sight.
IMAGE CREDIT: NASA, ESA, Harald Ebeling (University of Hawaii at Manoa) & Jean-Paul Kneib (LAM)