What difficulties did space immigrants encounter?

Now that human beings have mastered relatively mature space technology, traveling or even living in the universe is no longer a dream? Faced with the population pressure of the earth, scientists put forward the idea of emigrating to other planets in the universe? But this idea can't be realized for the time being, because it involves all kinds of extremely complicated scientific problems?

First of all, human beings and animals have undergone a long evolution in the earth's environment before they gradually adapt to the current physics on the earth? The living environment of chemistry? Once you go to live in space, the living environment there is very different from the earth? Even if the design is similar to the earth's environment, it is difficult to maintain it for a long time? Once the environment changes, the consequences are unimaginable?

Secondly, human beings live in space. Once they are attacked by meteors, will they escape in the air? Without air to sustain human survival, humans and animals can't survive?

Thirdly, it is not only expensive to build a human living area in space, but also difficult to achieve with the current level of science and technology? So, is it still difficult to realize the idea of human migration to space at present?