It was less than a century ago that most astronomers assumed that our Milky Way galaxy was the only galaxy in the universe. They supposed all that lay beyond our galaxy was an immense nothingness, an infinite void—empty, cold, and devoid of stars, light, and life.
As telescopes became more sophisticated—including telescopes that could be launched into space—astronomers began to grasp a spectacular,almost incomprehensible truth: the universe is mind-bogglingly bigger than anyone had previously believed, and the heavens are filled with numberless galaxies, unimaginably far away from us, each containing hundreds of billions of stars.
...before mankind had instruments powerful enough to gather celestial light and bring these galaxies into visibility, we did not believe such a thing was possible.President Uchtdorf goes on to explain that it's hard to believe in that which we cannot see. Though those other galaxies were there all along, we couldn't see them, so we didn't believe that they existed. In Why We Can't See Stars, I hope I made it clear that while I was talking about seeing God's light, I meant seeing evidence of God's existence and love for us, not literally seeing the light that God generates when He appears to people. Only a very few people in the world's history have been privileged to literally see God. The rest of us just have to trust that He really is there. Even when we do everything we can to cut out the distractions in our lives, humble ourselves, and try to attune our eyes to the light of God, we still probably won't see Him, and some measure of faith will always be required.
I do not know that other galaxies exist. I've been told that they exist, and I've been shown images of them, but I do not know of their existence for myself. Even if I was allowed to look through a telescope and see galaxies other than our own, I probably couldn't help thinking of how easy it would be to embed a computer screen into a device than merely looks like a telescope. There would always be some doubt, and I would always need some faith.
The same goes for knowing that God exists, that there is life after death, and that the scriptures and words of the prophets are true. The prophets and our own hearts may tell us that certain things are true, just as astronomers and telescopes can tell us about other planets and galaxies, but it will always take some faith to believe them. And some people have trouble coming up with that kind of faith. In fact, I think we all do sometimes. Sometimes, we can't bring ourselves to believe in things that we haven't observed for ourselves. Sometimes we feel that we need to see a thing ourselves to believe in its existence, and while that may be a wise course of action in some situations, like needing to personally see a platform before jumping halfway across a chasm to land on it (or miss it and fall to you death, as would happen if the platform wasn't really there), but some things are better when we accept them by faith, even if they aren't really there.
I'm pretty sure that God exists. Since there's always some need for faith and some room for doubt, it's hard to be 100% sure, but I'm pretty darn sure He's out there somewhere, and even if He isn't, I think it makes sense to believe in Him anyway. A belief in God encourages people to behave righteously. Believing in God generally makes people better people. Even if there is no God, believing in Him will help make the world a better place for everyone. It doesn't truly matter whether other galaxies exist. We're never going to reach them and I doubt we'll learn much from them, so whether they exist or not doesn't have much of an impact on us and our world, but believing in God, and encouraging others to believe in Him, will make life better for almost everyone. I haven't seen God. I don't know that He exists. But I'm going to act as if I do know, partly because it'll make me a better person, and partly because I'm still pretty darn sure that He exists.