The problem is that the sky is too limiting. We're not going to colonize any other planets any time soon, not even the moon or Mars. The planet is over populated to the extent that we're causing serious damage to the environment (the last estimate I saw said the planet was about 30% overpopulated) and we're all pretty much stuck on Earth for at least another century unless a miracle invention happens (and it would need to be a pretty big miracle).
Space ships have that pesky speed of light problem to get to other planets. It simply takes too long to get there. Our farthest probe is less than a half light day away from earth after traveling over 25 years. The nearest star is almost 4 light years away.
A generational ship is not feasible in the near future because at least 30 people are needed for a diverse gene pool (to prevent in-breeding problems) and we aren't close to transporting that many people at once anywhere off-planet. The total number of people who have reached the moon is only about one dozen. We have yet to even find a planet that we can make hospitable. The only hope so far is that we find actual water on Mars, not just evidence that there once was water on Mars. Even if that happens, we would have to create some sort of self-sufficient ecosphere on Mars even though we can't get it to work for the long-term on Earth.
Even if we someday get Star Trek transporter technology, we shouldn't use it. The E=mc2 equation tells us that a typical human body converted to energy would be about 20% of the world's nuclear arsenal in energy output. One mishap and we could become extinct. Transporter technology is at least 100 years away we estimate, if it's possible at all.
Maybe we can create a wormhole? It's been calculated that the energy needed to create a rip in space would be enough to end the universe.
Can we find a pre-existing wormhole? We've never even observed one before and we have looked really hard for them. It's just a theory.
A negative energy portal has been suggested. Negative energy in this case is less than the absence of energy (it is not electricity). The theory goes that if you can "owe" enough energy in a localized space, it will warp space enough to create a portal. We can create negative energy, but in tiny amounts and we don't even have a theoretical way to practically create such a portal.
Read more on Negative Energy, Wormholes and Warp Drive
Let's face it, as a race we're stuck on planet Earth for a long time to come. A few people may come and go, but outer space is not a colonization option for at least 100 years (and that's an overly optimistic estimate). We haven't even a viable theory to get an off-planet human colony started. Putting our population on Mars slowly would only help conserve our resources if Mars has enough resources to contribute to even a small colony. So far this is all speculation.
Let's face it, in our life times and our grandchildren's lifetimes, the sky will be the limit.