First and foremost, I will state that I am supposing that the creation of habitable environments on the large and small scale are achievable and that the human species will desire and succeed to achieve this. That is, we currently are not able to spread our species out amongst the stars, primarily due to the fact that we are currently unable to meet the challenges of long-duration interstellar travel.
There are three limiting factors to man's ability to extend out into the universe and they are: time, technology, and physiology.
In regards to time, human expansion into the universe is particularly limited currently. Outer space is huge. Colossally huge. I'd suggest that it is indeed the biggest thing possible. And in addition, space is for the most part completely empty. And I don't mean that there isn't discernible matter, but that those quantities of matter over the microscopic level, are quite few and far between. People don't even understand the magnitude of emptiness that space is, perhaps largely due to our inability to comprehend that which we have not or cannot experience first hand. Either that or bad sci-fi movies. Either way.
Because the universe is so big, we will see that there are several issues with man's expansion across it. The first is that matter can only travel up to approaching the speed of light. Even it it were possible to accelerate a person up to that level safely, I suggest that spending a hundred million years shooting from one end of the galaxy to the other is practically useless.
As a subset of time, we need to understand that the universal speed limit, the speed of light, dictates that there is a maximum speed at which we can travel without utilizing a time-devoid inter-dimensional method of transportation. (I suggest that such a method of travel is impossible, as I will state in a later post.) Therefore, there is a minimum amount of time it takes to get from one place to another. From the sun to Earth in under ten minutes sounds great. But over a year to our nearest neighboring stars? Thousands of years or even millions to other potentially inhabitable planets? There comes a point when the size of the universe inhibits man from being able expand and also remain in contact with each other. There comes a size after which the species will be so far out of contact with each other that the species will grow in separate directions. While that distance may not be a light minute or second. I would suggest it would be about the distance that light travels in 24 hours. At a point where it takes transmission, not even human transportation, 24 hours to travel in each direction, the desire of humankind to remain a cohesive unit, will likely deteriorate.
It should be noted however, that humanity will not stop spreading at this point, but that fractured segments of humanity will grow and flower in different areas of the galaxy. At this point, it is highly unlikely (without FLT) to leave the galaxy. Human lifespans and the sheer amount of space in between galaxies is a strongly limiting factor. We would simply lose interest either by design or unwillingly.
Closely attached to time, is human physiology. I have touched upon human lifespan limitations, but there is a further limiting factor to human expansion. It is the ability of the human body to endure acceleration. There is an upper limit to which the human body cannot any longer endure the force created from acceleration. Therefore, there are further time limitations involved in the velocity of space travel. Acceleration would have to be a much slower process for humans than for automated satellites (and so on). As would braking.
With the factors of the light speed speed limit and the inability of the human body to endure rapid acceleration and deceleration, we limit the ability of humanity to spread out into the universe beyond a heretofore unknown distance from a central point (say Earth).
The final and most obvious limitation of interstellar travel is technology. Mankind is unable to race between the stars even within the aforementioned limitations, because we lack the technology to do so. We lack the ability to travel fast enough, yes. But, also we lack the ability to produce food, recycle waste, and in essence sustain ourselves in small enclosed shuttle environments. Additionally, the affects of a lack of gravity and of radiation bombardment lead to similar difficulties, although the latter could likely be solved with present technology if the necessity of it arose.
So where are we going as a species? We will spread out into the universe provided we gain the technology to do so. How far we spread will be limited by our technology, by our human physiology, and by the simple vastness of space.
One may ask if there is other life out there. There very well could be. Right now. But it could be at such a distance that, one: we are unable presently to reach them or visa versa, or two: it is physically impossible for the two groups to come into contact in a mutually acceptable quantity of time. The fact is that like lost Amazonian tribes, we may indeed think that there is no greater world out there, simply because it is too far beyond our abilities to reach it or too far distant that the laws of physics themselves have prevented us from coming into contact. Likewise, as these factors play out, the physical distance between two entities makes it increasingly statistically insignificant that two separate groups will ever meet up.