There are two kinds of people in the world: people who use data as a plural noun and people who are scolded for not doing so. Who is right?
The word data is Latin for things that are given, the plural of datum. A datum is surely a singular thing, countable like a grape. This kind of noun is called a count noun. Saying the data is must sound as surely wrong to some people as saying the grapes is.
In the time of the Roman Empire, data were like grapes: you knew how many you had, and you were interested in each one individually. Do you have a datum on that, a centurion might have said when discussing something in a forum1.
These days, though, data is a bit more like rice—too many pieces to count. Millions of bits of data surround us constantly. Who knows how many grains of rice is2 in a serving? I eat grapes one by one, but rice is different: I don’t care about the individual pieces. It's more of a substance, even though it's got a granularity.3
That's the feeling of data in the modern world: so numerous as to be like a substance despite having a granularity.
We measure substances not by grains but by mass, and we call this type of noun a mass noun. Mass nouns don't have plural forms the way count nouns do. You would never say you wanted to eat a bowl of “rices." The word rice doesn't feel like a plural—and that's how the word data feels for people who consider it a substance rather than a collection of individual things. That's why it sounds as wrong to some people to say the data are as it would to say the rice are.
Only you can decide whether you consider data a mass noun or a count noun, but I think I know which way the data might be pointing.
Forum means what is outdoors, and referred to the outdoors assembly place in a Roman town where people could gather and talk about things. Today, of course, no one goes outside, so our discussions take place online in a different kind of forum.
Gotcha!
Granularity comes from the same word as grain, which rice is one of.