> If I uploaded a bunch of data that was obtained illegally there is nothing stopping me from doing that right?

The hosting provider is responsible for removing illegal content. Dat itself doesn't track any content. The datproject.org is more of a registry, not a host.

> Also, is your peer to peer network able to be attacked by nefarious users like a sybil attack? Is there a situation where I could alter or forge data?

No, only authorized people can write to each dat key (currently only the owner, but multi-writer is coming soon). All the writes are signed with the writers private key and then verified whenever content is downloaded.