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 reactive and combustible. Hydrogen fuel is a type of fuel that uses hydrogen as its primary energy source. This fuel can be used in fuel cells to generate electricity, or in internal combustion engines and turbines to produce mechanical power. Hydrogen fuel is considered to be a clean and renewable energy source because it produces no emissions when it is burned.