In Space...Without a Suit
Space is a dangerous place for humans. Our bodies evolved to the relatively safe Earth. In space, astronauts need to worry about things like cosmic radiation, breathing, and not boiling. As weird as it sounds, blood would boil in space.
It is analogous to a pressure cooker. The pressure cooker allows food to be cooked quicker because the higher the pressure, the higher the boiling point. In the vacuum of space, there is no pressure. This would make the boiling point in the blood to drastically decrease. When that happens, the blood boils, organs expand.
Luckily, the space suit was designed with this in mind. The space suit is filled with gas that pushes back on the astronauts body. This "push" help keep the air pressure higher and thus keeping the blood from boiling.
Aside from boiling, a suit-less astronaut would not be able to breath. They would last about 15 seconds. On Earth, people can hold their breath much longer than that, but in space, lungs full of air would be a bad thing. The air would expand and so would the lungs.
The dangers in space are avoidable and prevented with the space suit. Life, as scientist know it, cannot survive in the vacuum of space. Research and preplanning has helped keep astronauts alive and helped humans explore the unknowns of space.