For a long time we've talked about an "interstitial" space in the body that's basically everything that's not a blood vessel or cell, but it was more of an abstract concept than something you could see. But now, scientists think they've started figuring out the interstitium and that it's its own organ. see more here
The best way I can describe it is thinking about the interstitium as being those little bags of air that get shipped with your products in those cardboard boxes to buffer and keep things from getting damaged. And imagine they're all interconnected too, kinda like those little air bags. They think that maybe this is where tattoo pigment lies, this might be how cancer can spread, it might be how pain sensors are activated and it might be the secret as to why and how acupuncture works. For more info on that last part, check this out: Do We Finally Understand How Acupuncture Works?