The warm-hot intergalactic medium (WHIM) has been seen before, along incredibly overdense regions, like the Sculptor wall, illustrated above. But it’s conceivable that there are still surprises out there in the Universe, and our current understanding will once again be subject to a revolution. (Spectrum: NASA/CXC/Univ. of California Irvine/T. Fang. Illustration: CXC/M. Weiss)

We Just Found The Missing Matter In The Universe, And Still Need Dark Matter

Many hoped we could do without dark matter. On cosmic scales, the evidence has finally spoken.

For over 40 years, scientists have argued over dark matter’s existence.

The extended rotation curve of M33, the Triangulum galaxy. These rotation curves of spiral galaxies ushered in the modern astrophysics concept of dark matter to the general field. The dashed curve would correspond to a galaxy without dark matter, which represents less than 1% of galaxies. (Wikimedia Commons user Stefania.deluca)

Big questions arose from the motions inside galaxies, clusters of galaxies, and along the cosmic web.

The cosmic web is driven by dark matter, with the largest-scale structure set by the expansion rate and dark energy. The small structures along the filaments form by the collapse of normal, electromagnetically-interacting matter. (Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC))

From their gravity, we can infer the total mass in the Universe.

The matter and energy content in the Universe at the present time (left) and at earlier times (right). Multiple lines of evidence indicate that normal (atomic) matter can only compose 1/6th of the total matter in the Universe; the remainder must be dark matter. (NASA, modified by Wikimedia Commons user 老陳, modified further by E. Siegel)

Yet multiple sources indicate that only 15% of that mass can be baryonic: made of normal matter.

The density fluctuations in the cosmic microwave background provide the seeds for modern cosmic structure to form, including stars, galaxies, clusters of galaxies, filaments, and large-scale cosmic voids.(Chris Blake and Sam Moorfield)