Cosmology is full of surprises and black holes are definitely one of them. Roughly speaking, a black hole is a tiny core with enormous amounts of matter packed into it. There are no known forces that can prevent the collapse of this core into a singularity, but then, we don't know all of physics, so there may be such a force after all. Another feature of a black is its event horizon, the mathematical line drawn in space beyond which there is no return possible to the outside, not even for light. Once in, there is no escape, everything will fall onto its core. However, the inadvertent traveller (or planet) moving through this event horizon may not notice a thing. For all we know, we may have passed through an event horizon of some black hole out there (I presume that we would probably have detected this but I don't know the details of this.)
Here is another one of those questions that kept me awake as a child: "is the universe really infinitely big"? The very idea seems absurd. However the reverse is not better; if there is an end, what's behind it that thing that marks the end? At any rate, the current wisdom is that the universe is infinitely big and expanding at an ever accellerating pace. The ramifications of this idea are far reaching. Imagine you are walking on a road made of rubber. You have reached the speed of light, but the rubber is expanding under your feet at a pace faster then you. You are trying to get to the end of the road, but will never make it. In fact, the endpoint is slowly receding while you are running at your top speed. The stuff nightmares are made of really.
The same fate befalls the light from distant stars and galaxies trying to reach us, perhaps carrying the messages of alien civilizations. As the universe is expanding, fewer and fewer of those distant stars can communicate with us and disappear behind a black curtain, forever invisible. And the bad news is that the black curtain is drawing nearer every minute. At some point in the future, there will be just us and a small cluster galaxies (see http://www.nytimes.com/2007/06/05/science/space/05essa.html?_r=1&pagewanted=2.)
The irony is that we will then be locked up in our private bubble of the universe. Even if it wanted to, the rest of universe cannot send its information to us, and reversely, we cannot send our information to them. We we live on "causally connected islands" floating in the middle of vast expanses of emptiness. Sending messages (in the form of light) into this void is futile since space is growing faster there than light can travel. Our messages will travel forever, never getting anywhere. Like the inhabitants of ther Easter island, we are locked up. Pretty claustrofobic idea really. But fortunately, we still have an odd trillion years to enyoy the stars in the sky!