Spikes reply

The only possible aliens that could, would or will land on Earth are microbes - bacteria and maybe viruses.

The main problem is that of distance, both in light-years and time. 
As far as the size of the universe is concerned, I will agree completely that there have to be aliens. However, unless there are uncountable billions of them, they wouldn't be able to spread evenly in a concentric manner. Don't forget that the universe is 3D, so they would have to spread up and well as out and across. That takes a lot of resources.
'People' as well as minerals and fuel.

Even if they leave the original planet in many directions, they are going to be spread more and more thinly the further they go.
Look at an old wooden wheel. Imagine that the home planet is the middle and that each spoke is the course of a separate colony ship. As they start out, they're next to each other, but by the time they reach the wheel rim, they're quite far apart.
If you translate that into astronomical distances, we're suddenly talking about a humongous area of unexplored (and, to all intents and purposes, non-explorable) universe.

Going back to the problem with time. There simply isn't enough time. The universe is expanding, which means more distance between galaxies and solar systems. Not every planet is the same age as another; by the time the sun's light reaches us, the sun itself is another 8 minutes older. The supernova in the sky last night was the death of a star that died thousands of years ago. The stars we see in the sky above us right now may have already died, but we just haven't found out about it yet. The universal messaging service just ain't that quick. How do you think a space craft travelling at a speed less than that of light is going to fare? It's barely going to cross the galaxy, let alone the universe.

Also, genetics won't allow it. Evolution is slow, but it happens anyway. Unless you manage to invent some kind of universe-hopping system (wormholes etc, which means you won't be exploring the intervening space, so you lose out that way too), the species will be sufficiently different by the time they get anywhere like the next galaxy.
Don't forget that evolution depends upon the environment and the needs of the inhabitants to survive it. That means that each of those colony ships going in different directions are also gradually becoming different to each other.
Another thing on the genetic side of things; evolution isn't always a case of better, better, better. Your can only go so far. Insects cannot grow to be cat sized, cats can't breathe underwater and jellyfish can't fly.
Yet. Oxygen levels may increase dramatically, which would allow insects to grow larger. Cats may grow to like water, spend more of their time near water and eventually begin to evolve flatter limbs to help them swim better (after all, that's what happened when a group of bears gradually became whales), and jellyfish may evolve a shape that allows the wind to catch them from the surface of the sea and which them away from danger.
These changes aren't *better*. They're just more appropriate. A sea cat isn't better than a land cat, it's just different.

Ultimately, any space-going species needs to be able to plan for journeys lasting generations, millennia to find even a planet that is habitable for them, let alone to find a planet that has intelligent life with which they can communicate (or exterminate, whichever they find easiest).
In the end, that species if it wants to survive as long as possible, needs to start planning ahead so they can build a space craft the size of a moon that can take what will amount to the whole species around the universe, looking for resources. Maybe they will find a suitable planet before the species implodes (due to genetic, sociological and environmental pressures), maybe they won't. However, they're most likely going to become some massive hermit crab, carrying its home around with it and occasionally shucking it to find something more suitable for its current needs.

But that's just my opinion there at the end.