The unknown pervades the universe

By Aussiegirl

A fascinating article on the latest theories of what constitutes "dark matter", the 96% of the total mass of the universe that we can't see. I chose the opening sentence of the article as my title, because it seems to be true that whatever we investigate ultimately ends up in a mystery -- the mystery of existence itself. Here are two of my favorite quotes from scientists themselves on this mystery: The most beautiful experience we can have is the mysterious - the fundamental emotion which stands at the cradle of true art and true science (Einstein) -- Not only is the universe stranger than we imagine, it is stranger than we can imagine (Eddington)

Dark matter | Accidence and substance | Economist.com

Two possible explanations for the bulk of reality
The unknown pervades the universe. That which people can see, with the aid of various sorts of telescope, accounts for just 4% of the total mass. The rest, however, must exist. Without it, galaxies would not survive and the universe would not be gently expanding, as witnessed by astronomers. What exactly constitutes this dark matter and dark energy remains mysterious, but physicists have recently uncovered some more clues, about the former, at least.

One possible explanation for dark matter is a group of subatomic particles called neutrinos. These objects are so difficult to catch that a screen made of lead a light-year thick would stop only half the neutrinos beamed at it from getting through. Yet neutrinos are thought to be the most abundant particles in the universe. Some ten thousand trillion trillion—most of them produced by nuclear reactions in the sun—reach Earth every second. All but a handful pass straight through the planet as if it wasn't there.

[...] That number is tiny—0.00001% of the mass of an electron. But it is significant because neutrinos are so plentiful. While their mass is so small that neutrinos cannot be the sole constituent of dark matter, they have an advantage in that they are at least known to exist.

Darkness and light
The same cannot be said for sure of another possible form of dark matter being studied by a group of physicists in Italy. In a recent issue of Physical Review Letters, Emilio Zavattini and his colleagues at the National Institute of Nuclear Physics in Legnaro report an unusual signal in an experiment that goes by the unwieldy name of PVLAS. Like the good, sceptical scientists they are, the team has spent the past two years trying to explain the signal away—for example, as an artefact produced by the instruments. So far, however, they have failed. If the result continues to withstand scrutiny, it would appear to be evidence for an exotic new sort of fundamental particle, known as an axion, that could also be a type of dark matter.