Hmm I think you’ve asked something that physicists wonder about all the time! The truth is we don’t know, but current scientific evidence suggests the universe is expanding, and, over the past 5 billion years, the rate of expansion has actually been increasing. Based on our current knowledge of nature, only gravity can slow expansion, but currently we believe there isn’t enough matter in the universe to ever allow it to shrink and collapse into itself. In any case the timescale of this is over billions of years (the Universe is 13 billion years old and the Earth is 4.567 billion years old), so I wouldn’t worry! 😉
Like Anthea said we dont really know yet and it depends on the amount of matter in the universe. If there is enough matter gravity from it would pull the universe back into a big crunch. However the stuff we normally call matter only makes up 15% so what about the rest? Well it gets wierd because physicists think that the rest of it is made up of dark matter. This is matter we can’t see but we know must be there as we can see the effects of its gravity, such as through the way galaxies spin. While we dont know what this matter is it might be a new particel we have yet to discover.
So is this enough matter to make the universe go crunch? Well it gets stranger again as there is stuff called dark energy (physicists are clearly not the most creative with names!). This dark energy makes up nealy 68.3% of all the energy in the universe (mass counts as energy too). This energy works against gravity making the universe expand more but like dark mass we dont know what it is.
These dark forces will fight it out over wheter the universe expands or contracts and until we figure them out we wont know who will win!