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.
Big questions arose from the motions inside galaxies, clusters of galaxies, and along the cosmic web.
From their gravity, we can infer the total mass in the Universe.
Yet multiple sources indicate that only 15% of that mass can be baryonic: made of normal matter.
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.
Many nevertheless wondered: could normal matter be hiding — and gravitating — entirely without dark matter?
Scientists set out to measure all the normal matter in the Universe, including stars, planets, gas, dust, and more.
Only ~20% was within galaxies and clusters; about another 35% was found along filaments and in cosmic voids.
Still, nearly half the normal matter remained missing, assumed hiding in heated, intergalactic plasmas.
Missing normal matter was theorized: the warm-hot intergalactic medium (WHIM).
X-ray scientists finally announced evidence for the hot part of the WHIM in precisely the predicted amounts.
If the results are universal, the mystery is solved: the missing normal matter has been found.
The conclusion? Dark matter is absolutely necessary.
Mostly Mute Monday tells the astronomical story of an object, phenomenon, or process in images, visuals, and no more than 200 words. Talk less, smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.