Well. I don't really know what to make of this, and am not at all sure I should recommend it to others. But I think I will anyway. Because, whatever my reservations, I was thoroughly fascinated by it. One big warning, though: the story does not end, just as it did not end with series 1. There isn't even the sort of resolution with a few loose threads that satisfies the desire for an ending while pointing the way to a sequel. According to Wikipedia the third season is to be the last one, so I'm hoping that means the developers actually planned a coherent three-season plot which will have a reasonable ending.
The story only becomes more complex in series 2, and I have major doubts as to whether it makes sense. The pseudo-science makes even less sense than before: opening a barrel of nuclear waste might make those in the immediate vicinity pretty sick, but I really don't think there's any danger of it disrupting the very fabric of space-time. And the use of wormholes to serve as the equivalent of magic in sci-fi has gotten tiresome.
But the complex and confusing plot line is anchored by elemental human drama: parents, children, love, death, misunderstanding, mistakes, separation and reconciliation. Not really all that much of that last one, though, at this point.
The contradictions inherent in the whole idea of time travel are handled more imaginatively here than in time-travel stories I've encountered. However, that seems to be a confession of ignorance on my part: suspecting that something called "the bootstrap paradox" was not invented by the writers of this series, I searched for it, and found that it goes back at least to a 1941 short story by Robert A. Heinlein. The basic idea is presented in the show as this: you take an object back in time and leave it there. So it exists in the present because it existed in the past. But since it was only in the past because you took it there, it was never created. At any rate, the movements in time are so many and so complex in Dark that the result is either a brilliant juggling act or a big mess. Either way, it's awfully well done--well-acted, well-written, well-produced.
Actually I'm concerned that something that happened in the last minute or so of this season may seal the "big mess" verdict. I'll find out when I watch series 3.