Missing the subject?

The subject is a singularity.

Um so he sort of changed the structure of the sentence, right? "Singularity is at the heart of a large black hole".

And what's the difference between the sentences with and without this?

A singularity is at the heart of a large black hole.

What you have in the original quotation is a structure called " a subject-verb inversion". There are several patterns in English in which this structure is used. Here is a link which gives examples of 18 different patterns.

NugsoI guess the subject is missing

No. It's a matter of there-deletion after locative inversion.

There is a singularity at the heart of a large black hole.
At the heart of a large black hole there is a singularity. (locative inversion)
At the heart of a large black hole is a singularity. (there-deletion)

CJ

Ah, thanks for the correction and the link. I've already added it to my bookmarks.

Hi, CJ. Thanks. Is there any subtle difference between them? It looks like the last one is more likely to be written by a poet.

NugsoUm so he sort of changed the structure of the sentence, right? "A singularity is at the heart of a large black hole".

He just reversed the order of the subject and the predicative complement. There is no difference in meaning. The basic order you have there would be less likely than the reversed order or the versions with there.

I see now, thanks.

NugsoHi, CJ. Thanks. Is there any subtle difference between them? It looks like the last one is more likely to be written by a poet.

Inversion is a typical poetic device. You're right about that. But these inversions also occur in other kinds of writing as well.

CJ