Is this an abscess?

This is a lump that appeared within a week and at first I thought it was a tumour. However you can see the dark spot in the photos which appeared last time when he had an abscess. So I think it is an abscess but does anyone else think so too? I'm tempted to treat it myself though since when both my rats had an abscess they popped naturally in a week or so. I am not entirely sure how serious an abscess is. He doesn't have a cage mate right now so I don't know how the infection occurred unless the same area got infected although it was ages ago. Should I start doing warm compresses or should I go to the vets for aspiration to find out what it is first?

It does look like an abscess.
Abscesses can be extremely serious such as if on the face/head, associated with a tumour etc

Here is some information. http://ratguide.com/health/integumentary_skin/abscess.php
You can decide to treat it yourself …. but some do need antibiotics and a vet visit
Personally. because of location, I would make a vet appointment (I tend to err on the side of caution), and in the meantime would do warm compresses as discussed in the link
but lilspaz68 would know better then me

Thank you SQ, lilspaz68 suggested warm compresses and I am looking into whether antibiotics are needed or not.