Two obvious hurdles to interstellar travel, the speed of light limit and the time slippage inherent in the travel speeds necessary to reach out to the stars. At the least, mass conversion technology will necessary to power such travel. Who knows if such technology is possible? Assuming that some way to power up to the near light speeds necessary to “reach out and touch” other stars can be found, only very long lived creatures could hope to make even one round trip within the lives of all they leave behind. Who would be willing to make such a trip? Heinlein either dodged these questions or found extra dimensional work arounds. One quibble with the Dark Forest Theory; species capable of interstellar travel will need to have had centuries of cooperation to develop such high technology without wiping themselves out.