The observable universe is a big place that has been around for more than 13 billion years. Up to two trillion galaxies made up of something like 20,000 billion billion stars surround our home galaxy. When we look at these numbers, it is hard to imagine that there is nobody else out there. But before looking for our new best friends or worst enemies, we have a problem to solve: What are we actually looking for? Watch "What Do Alien Civilizations Look Like? The Kardashev Scale" now in French, Hin