What kind of cool and interesting stuff can be found in the space between galaxies?

Zero point energy (ZPE) may be the most interesting and a possible source of so called unknown dark energy or possibly a new energy source/mechanism for expanded space travel.The idea of an Aether property of space may be coming back. And some argue that ZPE may be a new free energy source while there is much skepticism for it.

https://en.wikipedia.org/wiki/Zero-point_energy

http://www.calphysics.org/zpe.html

"This disagreement between the theoretical and measured values of vacuum energy may be one the greatest unsolved mysteries of physics and is known as “The Vacuum Catastrophe.”

The Vacuum Catastrophe

The measured value of vacuum energy is 10¹²⁰ times less than the prediction made by Quantum Field Theory!"

https://medium.com/nakshatra/the-nature-of-nothing...

http://large.stanford.edu/courses/2017/ph240/blake...

The Hunt for Zero Point ...online book by Jane's Defense editor / military researcher Nick Cook on some history of Electro-Gravitic / anti-gravity research dating back to pre WWII and Nazi Germany which may involve ZPE.

https://www.salon.com/2002/08/05/zero_gravity/

https://www.bibliotecapleyades.net/archivos_pdf/hu...

gas, dark matter,

my mother in laws ship

Hope you brought a lot of reading material. At the speed we can travel which is extremely slow, it is going to be years of absolutely nothing.

There could be a lot of loose planets out there with no stars to orbit

How cool would you actually want it to be

The short answer --

We do not know.

The long answer --

We do not know.

Well actually, we do know that some stars (and their attendant planets) get ejected from their galaxy by galaxy mergers.

There may be life on a planet of such a star. And that life may have found out the meaning of life.

Too bad they won't be be able to tell anyone.

Possibly dark matter.

Possibly a vacuum that is orders of magnitude thinner than exists in our solar systemm

A few atoms of hydrogen here and there.

There are probably planets that were propelled out of orbit of their stars. They would be frozen and dark and hard to find.

Death Stars, alien motherships. etc

You have dark energy which is pretty much everywhere (if it actually makes some sense for it to be somewhere)

We don't know.