UK Air Date (BBC 2): September 21st at 7.45pm, US Airdate (PBS): November 3rd at 9pm

When we gaze up at the night time sky, we see the shining stars in the blackness of space. But is the space empty?

In the 1950s a young American scientist discovered that stars in rotating spiral galaxies seem to be held together by an unseen force. Vera Rubin proposed the idea that the space between the stars was filled with invisible stuff she called dark matter.

Scientists now believe that dark matter could make up an unbelievable 99% of the universe. The density of the universe determines whether it will expand forever or eventually shrink back in a reverse of the Big Bang, otherwise known as the Big Crunch.

Dark matter also controls galaxy formation. After the Big Bang, dark matter was the first stuff to coagulate and form pools, into which ordinary matter fell under the influence of gravity to form galaxies.

The most probable place for all that dark matter to be lurking is in the dark remnants of dead stars that orbit in the outskirts of galaxies. These have indeed been found, but not in sufficient quantity to account for all the dark matter.

The dark matter must be in particles that were made in the Big Bang but did not form stars and galaxies and all the rest of the stuff that we can see in the universe.

The first type of particle that was tested as a candidate for dark matter was called neutrino. This tiny burst of energy is released during radioactive decay. But so far we can not detect any mass for the neutrino. Without mass, they couldn't have any gravitational pull and therefore couldn't be the dark matter.

The next suggestion for the particle controlling our fate was cold dark matter. The search is on down a salt mine in North England, but so far they've found nothing.

Our universe if totally under the control of the dark matter. Yet we can't see it and we don't know what it is.

The race is on to find this mysterious stuff. As soon as we do we will know whether our universe will expand forever and end as a frozen desert or contract under the force of gravity in a terrifying Big Crunch.

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