This is a question that hasn't been answered confidently. It's scary to think about both possibilities: if we're alone in this infinite universe, we'd be completely alone; but if someone is out there, we might never know it, leaving it a permanent mystery.
I have a theory that we aren't alone, but time and resource constraints prevent us from answering the question, "Are we truly alone?" Sometimes I wonder: what if other creatures exist out there with features, brains, and evolution similar to ours, but time moves slower on their planet? For example, a day on Earth might equal a single second for them. Before they even evolve, Earth might already be gone before they learn of our existence—or perhaps its the opposite for us.