It is all around us; it fills the universe and yet we cannot see it, touch it or even define what it is. Astronomers the world over are still trying to explain this elusive presence in the Universe - a presence with nothing except its weight to prove its existence. So far all it has is a name: dark matter.

Today, in the last of the National Science Week 2002 Science at Seven lectures, Jeremiah Ostriker, Plumian Professor of Astronomy, will be explaining how we came to discover this mysterious element of our Universe and what it might tell us about our past and our future.

In the last 100 years, an extraordinary amount has been discovered about the universe. It wasn't until the 20th century that astronomers discovered that ours was not the only galaxy, but one of many. Later still we were able to use increasingly sophisticated telescopes that allowed us to see so far out into space that we can actually look back in time.

But this increased knowledge brings with it even deeper mysteries, as Professor Ostriker explains:
"Once we realised we could calculate the weights of planets and stars by measuring their gravitational pull, we started to realise that there was much more matter in the Universe than can be accounted for by the visible objects alone," he said.

"There are elements - particles of some kind perhaps - that don't shine, don't react with anything and yet they exist in the Universe. It seems to be their gravity that is the 'glue' that holds galaxies together.

"More recently still, we have discovered strong evidence to suggest that alongside this dark matter is an even stronger component of "dark energy", which dominates gravity and is causing the universe to expand at an ever accelerating rate. What this means for the future of this Universe is something we can still only guess at."

Professor Ostriker's talk will take place on 22 March, at 7pm in the Lady Mitchell Hall, Sidgwick Avenue. Admission is free and the doors will open at 6.20pm.

And don't forget that there is a whole programme of events celebrating National Science Week tomorrow (Saturday).

From 10am to 5pm there are free events for the whole family to enjoy. From a physics workout at the Cavendish Laboratory, Madingley Road to herbal remedies at the Botanic Garden, Bateman Street.

For more details call the information centre on 01223 766766 or look at the web site.

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