The question of why no extraterrestrials have been discovered so far involves many aspects, including scientific theories, observation techniques, and the possibility of extraterrestrial life.

First of all, the vastness of the universe is unimaginable to human beings. Even though life may exist in the universe as we know it, it is not easy to find another planet with intelligent life due to the size of the universe. Professor Howard Smith of Harvard University points out that even with a few clues, it is difficult to establish a connection with extraterrestrial life. From the perspective of probability, although there are a large number of planets, the conditions suitable for the existence of life are extremely harsh, and there may be very few planets that can support advanced intelligent life.

Second, technological limitations are a big reason why we haven’t been able to find aliens. Despite advances in our observation technology, current telescopes and detectors are not yet able to detect faint signs of life in distant galaxies, or they simply do not exist. In addition, signals that may exist for extraterrestrial life may be overwhelmed by cosmic noise, or there may be a huge time delay between sending and receiving signals, making it difficult for us to capture.

Again, our current understanding of the morphology and communication methods of extraterrestrial life may be very limited. Extraterrestrial life may adopt a completely different way of life than life on Earth, and its signals or signs of existence may be very different from what we perceive, making it difficult for us to identify.

Finally, there is also the view that extraterrestrial life may exist, but that they may not be able to establish contact with Earth for a variety of reasons, such as technical limitations, lack of interest in Earth, or some kind of isolation.

No definitive evidence of aliens has been found so far, and the reasons may be manifold, but here are some of the main theories and opinions:

1. The scale of the universe is enormous: the vastness of the universe makes the search for extraterrestrial life extremely difficult. There are tens of billions of stars in the Milky Way, and there are an estimated hundreds of billions of galaxies in the universe. Even within the Milky Way, planets are extremely far apart, making it much more difficult to discover and communicate.
2. Window of life: The conditions for the birth and development of life are harsh, requiring suitable temperatures, liquid water, a stable climate, and a complex set of chemical conditions. Although there are many “habitable zone” planets in the universe that may be suitable for life, not all of them have life, and the window for the emergence and evolution of life may be short.
3. Technological limitations: The current human search for extraterrestrial life has limited technical means and observation capabilities. It is only in the last few decades that we have the ability to detect the atmospheric composition of other planets and some signs that may suggest the presence of life, but these detection technologies are still developing.
4. The Great Filter Hypothesis: This hypothesis suggests that there may be one or more “filters” in the evolution from inanimate to interstellar civilizations, and that only a few species are able to overcome these hurdles. This could mean that either life itself is very rare, or advanced civilizations have gone extinct on their own before reaching the stage of interstellar travel and exchange.
5. Illegible signals: If extraterrestrial civilizations exist and emit signals, we may not have captured them or may not be able to identify them. In addition, the distance of time and space makes real-time communication almost impossible.
6. Civilizations are out of sync in their development: Possible extraterrestrial civilizations may not have developed to the stage where they are able or willing to engage in interstellar communication, or have already surpassed this stage and have opted for more advanced or covert means of contact.
7. Civilization lifespan and distribution: Extraterrestrial civilizations may have relatively short life cycles, or they may be scattered throughout the universe, far away from each other and unable to easily meet.

In conclusion, although we have not yet discovered extraterrestrial life, this does not negate the possibility of their existence. The search for extraterrestrial life remains an important goal of astronomy and the SETI (Search for Extraterrestrial Intelligent Life) project, and the reasons why extraterrestrials have not been discovered so far are varied, including the sheer size of the universe, technological limitations, and a lack of understanding of extraterrestrial life forms and communication methods. This does not mean that extraterrestrial life does not necessarily exist, but it may just that we have not yet found the right way and time to detect their existence. With the advancement of science and technology and more exploration, we may be able to solve this puzzle one day.