Did you read my blog post Humanity versus the Universe? The universe is huge. Unbelievably huge. And we can't visit, humanity is stuck near hear for the foreseeable future.
The biggest problem appears to be God, who has decreed, "Thou shalt not exceed the speed of light." Maybe she has a good reason, like keeping us from messing up the rest of her universe, but I'm still frustrated.
Ignoring problems of energy and conservation of momentum for the moment (see Inventions: Likely, Possible, and Damn!), interstellar travel simply takes too much time. Even local travel is slow. Assuming a 1-gravity acceleration, our own Oort cloud takes a year or more to reach. With today's technology, the Oort cloud is decades or centuries away. Other stars? The closest is a five-year journey even with our near-magical 1-G space drive. There are two-dozen stars within a dozen light-years, and five-dozen within about 16 light-years. Real travel times will be roughly the distance in years plus one.
One convenient factoid: one gravity of acceleration is approximately one light-year per year per year, and thanks to time dilation the perceived travel times roughly follow non-relativistic rules (as long as we don't care to return to home). A year of 1-G acceleration gets you close to the speed of light, and a second year gets you a little closer to c, but time passes that much slower (equivalently, the distance to your destination appears to shrink). So we can, in principle, travel considerable distances in the span of a human lifetime. Hundreds of light-years.
But even our Milky Way galaxy is large, roughly 100,000 light-years across, 30,000 LY to the core.
If we want to visit another galaxy, it gets worse. Andromeda (our nearest large neighbor and the furthest naked-eye object in the sky) is fully 2 million LY away.
It is difficult to imagine a civilization spanning travel times of years or decades, let alone millennia. I can imagine humanity spreading across the galaxy, perhaps eventually heading for other galaxies in our Local Group, millions of years hence. And that will lead to another post, on the Fermi Paradox. But there's another problem.
Let me correct my comment about humanity spreading to the stars: I see no way for humanity to do that, because it will be our descendants, who are not likely to think of themselves as human. The time frames and distances are so vast that interbreeding is completely impossible: we will have evolved into many species, all alien to each other, likely with less in common than homo sapiens has with Neanderthal.
So our remote, non-homo-sapiens descendants will inherit the universe. I hope. I'm still afraid that our computers will own the future, leaving us behind (see The Technological Singularity).
So here's hoping that God is just hiding the keys to the universe until we learn to behave well, to not soil our home, and to get along with others. Then once we've proven ourselves, she'll let us learn about practical wormholes, or warp drives, or hyperspace travel. While she's at it, perhaps we'll learn the secrets to gravity control, cheap, unlimited energy, and ways around those pesky problems of conservation of energy and momentum.
At least as a science fiction writer I can make all those problems vanish with a wave of my literary wand. Poof!