The best way to conceptualize this is to imagine all the stars and galaxies on the edge of a rapidly expanding balloon. This is why we can now detect things that are 46 billion light years away from us despite the universe only being 13.8 billion years old (http://www.space.com/24073-how-big-is-the-universe.html).
What is revealed to us with our most advanced instruments is limited to the speed of light, which is what makes up the observable universe. We can only speculate what is beyond this point.
With this in mind, at some point millions of years into the future, all the galaxies that we can now detect will have expanded beyond the reach of light. We will no longer see them or be able to detect them in any way under the known laws of physics. It will be as though they no longer exist.
What happens if life on another planet in a distant galaxy develops after this period of time? Suppose they reach the same level of scientific sophistication we have now. They could have all the equivalent methods, capabilities, and scientific understanding we have yet would amass entirely different data without ever knowing that something was missing.
In fact, for this future life form, all the best data would suggest they live in a single galaxy in the midst of an infinite empty space. Simply because of time, their understanding of the universe would be severely lacking yet perfectly credible given the circumstances.
WHAT iF we're that life form?
How do we even know what we might be missing? What information might no longer be available to us that would tell an entirely different story about the state of the universe?
What if our questions about why the universe is expanding would have been obvious in a prior time?