neutrinos... where do they 'end up'?

> where are they going and where do you suppose they'll 'end up'?

They're not going anywhere in particular. Once an Neutrino is created, it has almost no chance to take part in a particle interaction and therefore will most likely exist as a Neutrino forever, just flowing through space on its own. Because they are nearly massless and so small (in energy) they mostly just fly around and pass through things undetected. There is a very small chance a neutrino will interact with ordinary matter, but that chance is very tiny. Neutrino detectors require 50,000 tons of pure water water surrounded by more than 11,000 light detectors in the hopes of "seeing" only a handful of neutrinos every second. Once a neutrino is created, it will most likely never interact with anything again.

> are they part of the 'dark energy' expansion engine out there?

I don't think anyone thinks that. The neutrinos have no mechanism for causing space to expand. They can only interact gravitationally and through the weak nuclear force. Neither of these forces can cause space to expand like it is. There is a group of scientists that think Neutrinos could be a factor in Dark Energy though. While this seems unlikely, it is slightly possible. Only time will tell.

> if they came 'with' the creation of the Universe, would that have us suspect that a self-destruct was built in?

Not really. The Neutrinos have no ability to destroy or stop anything. They barely interact at all. In fact it wouldn't uncommon for a neutrino to travel through the entire Earth and not once interact with anything. Neutrinos are nothing more than the byproduct of particle interactions. Their entire purpose seems to be to force conservation laws to be true (and they don't always do that either).

Eventually they will probably interact with a nucleus and be absorbed, though that will take a very long time. They aren't dark energy, but they MAY be a part of dark matter though we don't think they're all the dark matter.

One theory is that they just get bigger.