There are more than 500 billion habitable planets in the Milky Way Galaxy. However, there is still no hope for mankind to come into contact with extraterrestrial civilizations in the near future. Moreover, there is no evidence to prove that such contacts could be possible in the past. Skeptics refer to the Fermi paradox: even if aliens exist, it is impossible for humans to meet them.
In 1950, US physicist and Nobel Prize winner Enrico Fermi was having dinner with his colleagues. The people were discussing the existence of extraterrestrial civilizations. Someone said that there were thousands of such civilizations in the Universe. "Well, where are they?" Fermi asked.
Indeed, imagining the size of the Universe is beyond human consciousness. However, we still do not have any facts to prove that we are not alone in limitless space.
British academician Michael Hart stated 25 years later that if other civilizations had existed, they would have contacted the human civilization a long time ago. Since such contacts never occurred, it means that extraterrestrial civilizations do not exist.
However, who said that intelligent aliens must communicate with human beings? Maybe they just do not have the technology for it. It is also possible that they simply do not want to communicate with us.
For the time being, there are five theories in the world that explain the reasons why the aliens have not come into direct contact with humans.
Theory 1. Other civilizations live in black holes.
The theory was coined by Vyacheslav Dokuchaev, a professor of the Institute of Nuclear Research of the Russian Academy of Sciences. According to him, there could be planets inside massive black holes. The planets rotate around the core - the so-called singularity - on stable orbits. The power of singularity gives them light and heat, which means that the planets could be habitable. However, the creatures that may inhabit them are unable to establish communication with planet Earth because it is practically impossible to overcome the black hole gravity.
Theory 2. There are very few other civilizations.
Carlos Cotta and Alvaro Morales of the University of Malaga, Spain, are certain that aliens undoubtedly exist. Just like humans, they send space probes to explore the Universe. Since no alien space probe has reached Earth so far, it only means that there are very few of such civilizations.
The researchers even made a mathematical model, which showed that if such a probe were designed to work for 50 million years, the number of extraterrestrial civilizations would vary from 100 to 1,000. If a probe were designed to operate for 100 million years, there would be not more than ten extraterrestrial civilizations.
Theory 3. Aliens are afraid of us.
According to Adrian Kent of the Perimeter Institute in Ontario, Canada, some space civilizations are in conflict with others for their natural resources. That is why, they may deliberately conceal their own existence from "competitors." The scientist believes that the "advertising," which the human civilization makes by launching space probes beyond the solar system, may eventually play a dirty trick on us as the probes may attract "colonizers."
Theory 4. Other civilizations have not reached the level of inter-planetary flights.
Humans have not been able to reach even Mars. It is not ruled out that extraterrestrial creatures have not reached the stage of the technological development which makes space flights to other planets possible.
Theory 5. The density of the distribution of intelligent civilizations in the Universe is very low.
This point of view is shared by Reginald Smith of the Bouchet-Franklin Institute. The scientist concluded that if a civilization searches for other forms of life for 1,000 years (we've been doing it for nearly 100 years), then the number of the civilizations that live in the area of the Milky Way Galaxy, may not be larger than 200. The planets, where they live, are situated so far from each other that it is practically impossible to exchange signals between them.