subject verb agreement - A sentence from Swan's PEU using plural 'are' and not singular 'is'

In Swan's PEU (3rd Edition), an entry number 157 of 'discourse markers' reads...

There are a very large number of these 'discourse markers', and it is impossible to give a complete list in a few pages.

I'm finding it difficult to understand. Why is there a plural verb are and not is? I also note that discourse markers are defined in quotes ('...') which means it's considered as a single entity.

There are a very large number of 'X' out there does not look better to
There is a very large number of 'X' out there

Let me quote a quote here

Despite the digital age, there is a very large number of venues and spaces that are looking for plays, and many of them are looking for new plays.

Also, a result from the Google Books

What makes the fields 'classical' is the fact that there is very large number of sources contributing.

What am I missing?

Answer

CGEL (The Cambridge Grammar of the English Language) discusses (at 3.3, p.349) situations where quantification is “expressed by means of a noun as head with an of PP as complement”. In these constructions terms like a lot, lots, a great deal, plenty, oodles and a number are in syntactic form the heads of the NPs in which they occur, but the semantic head is the ‘oblique’, the term which is the object of of:

a. Lots of work^oblique is left to be done.
b. A lot of people^oblique were present.

Note that in a lots is plural, but the entire NP takes a singular verb, is, because its oblique, work, is non-count, while in b a lot is singular but the entire NP takes a plural verb, were, because its oblique, people, is plural-only.

In these cases, says CGEL, “lot is number-transparent in that it allows the number of the oblique to percolate up to determine the number of the whole NP.”

In other words, semantics trumps syntax with these “number transparent quantificational nouns”: the verb agrees with the sense rather than the form.

In your sentence, consequently, the verb correctly takes the plural because its real subject is the oblique these 'discourse markers', not a very large number.

With many singular number-transparent nouns such as number, however, many people feel uncomfortable with a violation of syntactic concord. Consequently, you are just as likely to encounter a 'number-opaque' treatment. In the circumstances, neither can legitimately be labeled “incorrect”.

Note, by the way, that the phrase a very large number acting as a NP on its own, not quantifying an oblique, will take a singular verb:

A small number is expressed in words; a large number is expressed in digits; a very large number is expressed in exponential notation.