"a massive audience" is a correct and usable phrase in written English. You can use it to refer to a large group of people, such as when describing the viewers of a live performance or a broadcasted event. For example: "The performers put on a spectacular show that drew in a massive audience.".
It had everything: artistic credibility, high drama, a massive audience.
"It's a massive audience, and they are extremely engaged.
Messrs Brin and Page grasped that their search engine could create a massive audience for advertisers.
"There's a massive audience for the spoken word, including in the MTV generation.
There's now a massive audience for the experimental outputs of the art laboratories.
They're not that fussed about TV deals for their animation because they don't need them to reach a massive audience.
"This is the interesting thing: we're creating a massive audience, but maybe the traditional people are not getting it.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com