There is a theory, which we got from Pythagoras, that the Earth's counterpart is hiding behind the Sun - Counter-Earth, or Gloria. Moving in their orbits parallel to each other, the twin planets are separated by the Sun, and therefore we, for example, cannot see Gloria, as well as its inhabitants, if any, us.
NASA officially denied the existence of Protype-Earth, sending two STEREO satellites into space in 2006, which circled the Sun in search of Gloria. Having monitored the area behind our Luminary, the devices did not find there even traces of the Earth's twin, with the exception of comets, and even then of small size. Therefore, it is logically possible to put an end to this question - there is no Gloria.
But, no, some independent researchers disagree with this statement. The fact is that Pythagoras and his students described the existence of this twin of the Earth too conclusively, but where did this twin disappear after two and a half millennia?
There are several theories about this. Some astronomers argue that during this time Gloria could well have left orbit and went to travel through large space. Others believe that Anti-Earth, observed by Pythagoras, is nothing more than another planet-nanagrah, along with Rahu and Ketu - mysterious objects of our solar system that exist on a more subtle plane than the planets we see. For this reason, the STEREO satellites could not find Gloria, but this is not yet proof of her absence in our solar system.
It is possible that the Earth's twin is being used by aliens for an invisible presence near us, but it is also possible that some of the aliens we encounter are actually just inhabitants of Gloria, where civilization has reached a higher level of development, which allows them to move between worlds. Well, why no one comes into contact with us is another topic, unfortunately, not very pleasant for us, but, I think, understandable by all of us.
In general, there is a lot of fog about Gloria. Skeptics, of course, exclaim in the famous words of Gorky - was there a boy? Enthusiasts appeal to another saying - there is no smoke without fire …