The alien civilizations: where are they?

The universe has a radius of fifteen billion light years. It contains a hundred billion galaxies, each containing up to two hundred billion stars. It is almost impossible that an inhabited planet revolves around just that one star, our sun. It would be very strange if we were the only intelligent species in the universe. But where are they, the extraterrestrial civilizations? For decades now, our telescopes have been scanning the heavens for traces of extraterrestrial technology, for messages from distant companions. So far without success. Do they exist? What do they look like? Like reptiles? Like insects? Like robots? Like gods? Like... people? Are they dangerous to us? Do they like us raw or fried? Can they help us? Or would we be all alone in a dead universe? This universe, about which the American scientist ].BS Haldane said that it is ''not only stranger than we imagine, more stranger than we can imagine.
In this book all possibilities are considered and investigated. With an open mind, without limitations and yet based on scientific arguments.