That's hard to say from that image. It could well be dust on the inside of the element, but it could well also be fungus.

The third possibility is that it could be scratching from overzealous cleaning in the past. I've seen this more times than fungus on lenses, and if that is the case there's nothing that can be done.

Do you have a lens blower? If so, I would advise giving the rear element a really thorough blasting with it, and see if it makes any difference at all (I suspect it wont in this case).

Zoom lenses are sometimes prone to picking up dust which creeps in between the telescoping sections of the barrel, which is what this could be.

Depending on how comfortable you are with working on a lens, this could be an easy one to rectify, or at the very least diagnose further.

The rear element should come out without much fuss, and if you use the blower on the inner surface of the element, and dust will be cleared out pretty effectively. If you do go down this route, there are some precautions which are worth taking.

Firstly, work in as dust-free an area as you can.

Secondly, wear something that doesn't drop lint or dust itself, so woollen sweaters are best avoided.

Thirdly, get some vinyl gloves to handle the lens element, and hold it only by the very edges. If this is just dust, the blower will remove it. If you put a fingerprint on the lens, you'll have to fully clean it which is a more tricky process.

With the element removed, use the blower to clear off any dust from the surface, and also use it on the inside of the lens before putting the element back in.

If this is indeed fungus, you can clean it using hydrogen peroxide, or a soap-free lens wipe (specifically the ones designed for camera lenses, not spectacles).

I would urge caution if you do undertake a cleaning yourself. Only ever apply the most gentle pressure with cleaning cloths, if you're using lens tissues use a new one each time you wipe the lens and throw the old one away. If you're using a washable lens cloth, use a different part of the cloth for each wipe of the glass, again use only the most gentle pressure, and wash the cloth after each cleaning session.

I've had some success removing very bad fungus from a lens using ordinary dish detergent, but this was on a very old lens, and there was nothing to lose by trying it. I do not know if this will be safe on new lenses or whether it will damage the coating.