Just caught up with it on Iplayer. I thought it was great, lush and I'm hoping it will get even better. I didn't like Lyra though, not sure why but she didn't seem very likeable to me.
I think Dust is supposed to be "dark matter" IRL. It represents 'knowledge', that is, the real hidden truth of the universe that the magisterium (which looked brilliant by the way), wants to keep hidden from the people. It was written basically as a critique of the Catholic church.. The M want it hidden, because once the people have the knowledge of the other worlds they would have 'free will'. Or something like that.
And it's all based on real quantum physics.
I could be wrong, I haven't read the books in over 10 years so can only vaguely remember what it was all about.