Although hydrogen is the most abundant element in the universe, it is not commonly used to fill balloons due to its highly flammable nature. When mixed with air, hydrogen gas easily forms a highly explosive mixture, posing significant safety risks. Even minor electrical discharges, including static electricity, can ignite hydrogen, causing a rapid combustion or even an explosion. In contrast, helium, though more expensive, is non-flammable and non-toxic, making it a much safer choice for filling balloons.