First detection of the missing half of normal matter in our universe

not exactly. As photons are constantly moving at the speed of light (citing the theory of relativity I don't really understand) they have no resting mass. But they have an impuls and create a force of impact (used in sun sails) from which a theoretical notion of ... mass (kinetic, perhaps?) can be derived. I'd like to know how it's derived originally, too though.

edit: if E=mc² with E(photon)=f•h and f=c/λ, then m=h/(λc) in vacuum. ... I hope that's correct. What's interesting, [m]=J/[a]/m

Wow, thanks a lot for this !

That's not it. We can figure out what the mass makeup of the Universe is through various independent means, and those are also independent of direct observation. Those lines of evidence tell us the Universe is a 69:26:5 split between dark energy, dark matter, and ordinary atomic (baryonic) matter. None of that has changed.

However, we've only been able to actually detect much less than that 5% contribution of the atomic matter in the Universe, meaning that we expect there's a lot more ordinary matter out there that we can't see. This research indicates that a big chunk of that missing ordinary matter is in the form of huge warm gas clouds on galactic scales.

Why should vacuum expand?

Is it due to gravity of the filament molecules pulling those molecules together into larger 'strings' or 'planes' which leave 'holes'? Or some other mechanism?

> Let's get this out of the way, the title is click-bait and the paper/researchers makes no such claims as to anything near 50%. New Scientist is trolling for hits with the word "half" or the journalist is fundamentally misunderstanding the work.

At first I agreed with you, but I've dug into the articles and re-read the New Scientist article too to make sure, and it seems the story is a bit more complicated than it at first appears (caveat: I'm also not a cosmologist). They should have clarified this research does not involve dark matter though.

Part of the confusion stems from losing context and awareness of implicit limits to the claims when translating exact cosmological terms to popular science. "Baryonic matter" means nothing to the average person, and calling it "observable matter" could also be confusing to lay-people, since this matter isn't actually directly observable:

> “There’s no sweet spot – no sweet instrument that we’ve invented yet that can directly observe this gas,” says Richard Ellis at University College London. “It’s been purely speculation until now.”

However, the researchers do seem to claim they solved the mystery of the missing observable matter by detecting gas filaments:

> “The missing baryon problem is solved,” says Hideki Tanimura at the Institute of Space Astrophysics in Orsay, France, leader of one of the groups. The other team was led by Anna de Graaff at the University of Edinburgh, UK.

Whether that can should be translated as "finding the missing 50% of observable matter" depends on whether those baryons are in fact 50% of missing observable matter. To make things more confusing for non-cosmologists here, the two papers tell slightly different stories, because they don't do the exact same thing. De Graaff's paper mentions a much lower number than 50%, as you stated, but the introduction of Tanimura mentions:

> At high redshift (z ≳ 2), most of the expected baryons are found in the Lyα absorption forest: the diffuse, photo-ionized in- tergalactic medium (IGM) with a temperature of 10⁴ – 10⁵ K (e.g., Weinberg et al. 1997; Rauch et al. 1997). However, at redshifts z ≲ 2, the observed baryons in stars, the cold interstellar medium, residual Lyα forest gas, OVI and BLA absorbers, and hot gas in clusters of galaxies account for only ∼50% of the expected baryons – the remainder has yet to be identified (e.g., Fukugita & Peebles 2004; Nicastro et al. 2008; Shull et al. 2012). Hydrodynamical simulations suggest that 40–50% of baryons could be in the form of shock-heated gas in a cosmic web between clusters of galaxies.

It looks like this is where that half in the New Scientist title comes from: 40-50% of missing baryons should be in these gas filaments. This might appear to contradict De Graaf et al., but the latter mention Tanimura et al. in the conclusions of their paper:

> Similar conclusions to this work have been independently drawn by Tanimura et al. (...) who announced their analysis (...) at the same time as this publication. (my summary: We used different, independent but complementary galaxy pair catalogues). Despite the differences, we achieved similar results in terms of the amplitudes and statistical significances of the filament signal. (...) The fact that two independent studies using two different catalogues achieve similar conclusions provides strong evidence for the detection of gas filaments.

So given that these two groups seem to be in agreement with each other's conclusions, and that Tanimura himself was quoted (so presumably consulted for the article), it seems that the main clickbait aspect of the New Scientist article is that they did not clarify that no dark matter is involved in this story.