96% OF OUR UNIVERSE IS MISSING - WHAT CAN THE MATTER BE?

ByANDY FLEMING

It's an embarrassment of gargantuan proportions that lies at the heart of modern physics, a kind of cosmic elephant in the room. Put simply, physicists realise that when we look out 13.7 billion light years across the visible universe with our telescopes, whether at visible, infrared, gamma ray or x-ray wavelengths, we are only seeing a tiny proportion of all that there is. Modern physics and its key theories of Newtonian and quantum mechanics and general relativity, which have successfully provided us with everything from iPods to GPS systems, simply doesn't have a clue as to what makes up 96% of the universe.

The best estimates of cosmologists and physicists reveal that only 4% of the universe is constituted of normal baryonic matter, consisting of the things we see with our eyes and

detectors. This is made up of atoms and their constituent parts -- and includes stars, planets

and intergalactic dust. Einstein said that mass and energy are equivalent, and since the late

1990s astronomers and cosmologists have found that a staggering 73% of the universe is made of something called Dark Energy, which reveals itself by an anti-gravitational force. 

It turns out that the expanding universe as first revealed by Edwin Hubble isn't just expanding at a linear rate; the expansion is accelerating. One day in the far and distant future, cosmologists will no longer see galaxies outside our own cluster -- they'll simply be over the horizon, too far away for light to have had enough time to travel to Earth. For now, though, we have little idea as to what Dark Energy actually is.

We may have rather more success in identifying Dark Matter, first postulated by astronomer Fritz Zwicky in 1934 to account for the 'missing mass' needed to sustain the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of Dark Matter in the universe, including the rotational speeds of galaxies,gravitational lensing of backgroundobjects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. It is believed thatmost Dark Matter, by its very nature, does not consist of atoms. It doesn't interact with electromagnetic radiation, and therefore we cannot detect it withour telescopes.

There are are many possibilities as to what Dark Matter may be, including the following:

 normal matter that has so far eluded our gaze, such as dark galaxies, brown dwarfs,

planetary material (rock, dust, etc.) or black holes. Some of these could be MACHOs

(Massive Astrophysical Compact Halo Objects), which would explain the distribution of Dark Matter in galaxy halos;

 massive standard-model neutrinos;

 massive exotica. These can be divided into two possible classes: 

* axions (hypothetical elementary particles), additional neutrinos, supersymmetric particles, or a host of others. Their properties are constrained by the theory that predicts them, but by virtue of their mass they solve the dark matter problem if they exist in the correct abundance;

*  particles with unspecified properties, but that are merely required to be massive and to have other properties such that they would so far have eluded discovery in the many experiments that have looked for new particles. Possibilities include WIMPS (Weakly Interacting Massive Particles), CHAMPs (Charged 

Massive Particles).

Whatever Dark Matter turns out to be (and there are many experiments being conducted around the globe to detect it, including at the Large Hadron Collider at CERN and in subterranean laboratories such as the one at Cleveland Potash's mine at Boulby, Whitby in the UK), we are likely to have an answer as to what this fundamental constituent of the universe is, long before that for Dark Energy. Whichever way you look at it, it's an embarrassment for modern physics to only know what 4% of the universe is actually made of!

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