Uncountable (mass) nouns refer to substances, concepts, or general terms for classes of items. The following words are uncountable nouns: oxygen (substance), education (concept), equipment (general term).
Do not use an article with an uncountable noun if you are referring to the noun in a general sense.
Marie and Pierre Curie discovered the radioactivity in
Marie and Pierre Curie discovered radioactivity in 1898.