God abhors a naked singularity.
Stephen Hawking1
A neat way of describing the cosmic censorship hypothesis, by Roger Penrose, which states that a singularity can only be seen from the inside of its own event horizon. That doesn't mean that they can't be seen: just that if you're brave/stupid/well insured enough to go in and have a look, then you ain't going to be coming back to tell us...

Stephen Hawkings lost a bet on this subject; his opponent showed that under certain (highly improbable and unstable) circumstances a singularity could exist without an event horizon forming. Under the terms of his bet, he had to 'clothe their nakedness'.

For further details, I suggest you read A brief history of Time by Professor Hawkings himself.

1: sometimes attributed to Roger Penrose. Anyone know which is right?