The term 'light bulb filler' refers to a gas that is used to fill the inside of an incandescent light bulb. The most common gas used for this purpose is ARGON, which is a colorless, odorless, and non-toxic gas. ARGON has five letters, making it the solution to the crossword puzzle. ARGON is used in light bulbs because it is an inert gas, meaning it does not react with other elements, and it helps to extend the life of the bulb by preventing oxidation of the filament.