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)

If there were more, the:

temperature imperfections in the cosmic microwave background,

galaxy correlations in large-scale structure,

and abundances of the light elements,

would be different.

The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. This indicates that 5% of the total energy density, and ~15% of the total matter, is in normal matter, and no more. (NASA / WMAP Science Team)

Many nevertheless wondered: could normal matter be hiding — and gravitating — entirely without dark matter?

An illustration of a slice of the cosmic web, as viewed by Hubble. The missing matter we can detect through electromagnetic signals is the normal matter alone; the dark matter is unaffected. (NASA, ESA, and A. Feild (STScI))

Scientists set out to measure all the normal matter in the Universe, including stars, planets, gas, dust, and more.

A 3D, reconstructed map of the total mass distribution in the cosmos. There wasn’t enough normal matter to account for this, so new search techniques needed to be devised to discover where, and how much, normal matter is truly, totally out there.

Only ~20% was within galaxies and clusters; about another 35% was found along filaments and in cosmic voids.

The formation of cosmic structure, on both large scales and small scales, is highly dependent on how dark matter and normal matter interact. Despite the indirect evidence for dark matter, it’s vitally important to count up all the normal matter and make sure it cannot account for what’s assumed to be missing. (Illustris Collaboration / Illustris Simulation)

Still, nearly half the normal matter remained missing, assumed hiding in heated, intergalactic plasmas.

A depiction of hydrogen gas within the intergalactic medium, or IGM, with bright areas indicating high gas density. (Vid Iršič)

Missing normal matter was theorized: the warm-hot intergalactic medium (WHIM).

Astronomers have used ESA’s XMM-Newton space observatory (lower right) to detect the WHIM. The white box encloses the filamentary structure of the hot gas that represents part of the WHIM. It is based on a cosmological simulation extending over more than 200 million light years. The red and orange regions have the highest densities & the green regions have lower densities. The oxygen detection is how the baryon abundance was reconstructed. (Illustrations and composition: ESA / ATG medialab; data: ESA / XMM-Newton / F. Nicastro et al. 2018; cosmological simulation: Princeton University/Renyue Cen)

X-ray scientists finally announced evidence for the hot part of the WHIM in precisely the predicted amounts.

The light from ultra-distant quasars provide cosmic laboratories for measuring not only the gas clouds they encounter along the way, but for the intergalactic medium that contains warm-and-hot plasmas outside of clusters, galaxies, and filaments. The X-ray emission from quasars enabled this newest detection by XMM-Newton. (Ed Janssen, ESO)

If the results are universal, the mystery is solved: the missing normal matter has been found.

By examining stars, dust, and gas in galaxies and clusters, scientists had found only 18% of the normal matter. But by surveying intergalactic space, including along filaments and in cosmic voids, scientists found not only gas, but ionized plasmas of all temperatures, that lead us to 100% of what’s expected. There is no more; and therefore, dark matter is still absolutely necessary. (ESA)

The conclusion? Dark matter is absolutely necessary.