Hydrogen and Helium are both elements and gases. Hydrogen is the most abundant ELEMENT in the universe, making up about 75% of all matter. It is a colorless, odorless, tasteless GAS that is highly flammable. Helium is the second most abundant ELEMENT in the universe, making up about 24% of all matter. It is also a colorless, odorless, tasteless GAS that is non-flammable. Both hydrogen and helium are used in a variety of applications, including rocket fuel, welding, and medical imaging.