A standing joke with astronomers is that we are only interested in hydrogen and helium – the two most abundant atoms in the universe… and every other type of atom (which all together make up less than 1% of the total) is classed as a “metal.” 
But there has been a lot of effort put towards detecting molecules from space – and with a good degree of success. Many of the molecules detected so far have been found in clouds of gas, in particular Sagittarius B2, a giant cloud near the centre of the Milky Way Galaxy. Some pretty complex molecules have been detected, including a simple sugar: glycolaldehyde and other molecules such as anthracene and naphthalene. The difficulty for these molecules lies in the fact that they are easy to break apart with ultraviolet light or x-rays – and space is full of that type of radiation. So molecules will be more stable if they are in the middle of large dark clouds – precisely the type of clouds that may go on to form new stars and planetary systems.
Interstellar chemistry is very different from Earth based chemistry. On Earth, many reactions take place in water, but in space it seems that reactions can take place on the surface of tiny dust grains. In this model of space chemistry, smaller molecules such as water, formaldehyde, methane or ammonia, coat the surfaces of dust grains in the clouds. If a shock wave from a star forming hits the dust grains, it provides energy to assemble more complex molecules from the simpler ones. Once the shock has passed, the molecules cool into a cold, thin gas.
CW Leonis as seen by SPIRE and PACS, with the emission lines due to water also shown.Image credit: ESA / SPIRE / PACS / MESS Consortia
Besides gas clouds, where conditions are cool, molecules have been found around stars, in particular older stars like CW Leonis. This star is relatively near (about 650 light years away) in the constellation of Leo. About 50 molecules have been detected in the vast cloud of material around it. Surprisingly water vapour has been found as well. This is unexpected because the star is a carbon star — nuclear fusion reactions deep inside the star are converting helium into carbon, much of which has ended up in the outer layers of the star’s atmosphere. It was expected that any loose oxygen would have bonded with the carbon to form carbon monoxide. When water was first detected, it was suggested that the star’s heat could be evaporating comets or even dwarf planets to produce the water. Recent observations of the infra-red spectrum of the star have shown that the water is being produced closer to the star than expected – down near its surface. In a previously unknown chemical reaction, ultraviolet light from interstellar space is breaking up the carbon monoxide and releasing oxygen atoms that can then react with hydrogen to form water molecules.
Molecules have also been found quite close to home – within comets of our solar system. The fabulous Hale-Bopp comet of 1996-97 was so bright that it was extensively studied and seven new molecules were spotted in that comet at the time. Given that some scientists suspect that many molecules on Earth were brought here by comets, they could be the connection between interstellar space and life on Earth.