Taking a closer look at the unseen universe

The project sounded tedious but straightforward—to find out how matter is distributed in spiral galaxies.

One approach was to carefully count the stars at various distances from the centre and to estimate how the mass of dust and gas is distributed around the galaxy.

Astronomers also tried a second approach. The speed a star moves in its orbit around the galaxy is dictated by the amount of mass lying within its orbit. We just move out from the centre, measuring the speed with which the stars are orbiting.

In a difficult project like this, we would not expect the two very different approaches to yield exactly the same result, but they should agree within some reasonable margin of error. They didn't.

The mass estimate using star velocities was around 20 times bigger than the mass obtained by counting stars and measuring dust and gas clouds. It was clear that a lot of stuff, in fact most of the material making up the galaxy, was not being seen.

Moreover, measurements for other galaxies yielded the same discrepancies. Someone called that invisible matter “dark matter", assuming that more careful and perhaps different observation would show it. However, despite dark matter's gravitational effects being obvious, nothing has been found. It remains unseen.

Things got worse when improved instruments and observing techniques made it possible to look for larger-scale structures in the universe.

We have known for decades that our galaxy, the Milky Way, along with the Andromeda Galaxy and more than 50 others, are members of a cluster of galaxies known as the Local Group. This, in turn, is part of a huge collection of galaxies known as the Virgo Supercluster.

We see other clusters and superclusters out in space as far as we can see. A reasonable assumption is that we would see these clusters and superclusters of galaxies strewn more or less randomly across the universe. However, that is far from what was found.

When researchers made careful measurements of the distance and position of thousands of galaxies and then plotted them to show their positions in space with respect to the Earth, something very odd emerged.

Galaxies, clusters of galaxies and superclusters of galaxies are not randomly scattered. The universe seems to be rather like a sponge, with galaxies forming strands and surfaces around great, empty voids. This structure is now often referred to as the "cosmic web".

It turns out that if we construct a computer simulation of a large volume of space with lots of galaxies in it, interacting because of their gravitational pulls at each other, they don't all fall together into one lump. They form a web of strands and surfaces, surrounding empty voids.

However, to get a result matching what we are observing, we need to add that dark matter, in the same proportion to real matter as we deduce from our observations of galaxies.

The implication is that "normal matter", which makes up our Earth and every living thing on it, the Sun, other planets and all the objects we observe across the universe constitutes only 5% or so of what is out there.

Nobody can possibly be happy with the need to correct our calculations by adding something that gives us the right answer but is otherwise totally unobservable.

So, understandably there is a huge effort to find out what dark matter might be, and, just as important, to detect it in some way other than just seeing its effects on things made from "normal matter".

One possibility is our understanding of gravity is wrong, and that over huge distances and times it behaves differently. Maybe, mistakenly, we don't think this is the case, so we are back to having to accept that at the moment we live in a universe where almost all of it remains unseen and not directly observable.

That is a very sobering thought, and one that has a lot of astronomers and physicists extremely busy.

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• Venus shines brilliantly in the southeast before dawn, with Mars nearby and Saturn low in the glow.

• The Moon will be full on March 17.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.