There have been some amazing answers to this question, and it's a fun one to ponder, so I will take a stab at it.
I like to try to relate science-fiction type environments to real-world constructs. I think they are easier to imagine, explain and dramatize in a story setting. With that in mind...
I imagine this boundary would be very much like a seam, like what one might find when two different pieces of fabric are sewn together in a whip-stitch pattern. The "pieces of fabric" in this case would be our universe (identified via elements of our laws of physics), the fabric of another universe (identified by areas where the laws of physics go haywire because the other universe has a different set of laws), and bound by the following three "threads:" 1) Time 2) Space 3) Light. This is where things get a little bit creative, a little more fiction than science.
Visually, this would appear like a corkscrew pattern running through a section of space where strands of pure light, pure space, and pure time were wrapping around, consistently digging into something, a void, as it were, where the laws of physics shift to reflect the neighboring universe.
If you were to travel across this pattern, it might go something like this:
1st Stop: Pure light. Can't get right inside this area because it's devoid of space and time, and I need both to continue to exist, so I'll just hover above and try to take some readings from this section of reality. Maybe I can learn a thing or two about light that I don't already know.
2nd Stop: Pure space. Still can't get inside, because I need time to exist but nothing moves. Absolutely nothing. It is a solid brick of space- it's not a black hole, cause there's no gravity well, but reality there is infinitely dense. 781
Let's ping off this section of reality and see what we find, shall we?
3rd Stop: Pure Time: Constant, frenzied frenetic fluctuation. Too chaotic; I'd be every when if I entered this string - simultaneously experiencing every moment of reality all at once. My head would explode.
4th Stop: Our universe - Typical laws of physics, everything is normal. Space, Time, and Light all co-mingle in a familiar harmony. I could hang out here all my life long if I have enough resources.
5th Stop: Pure Light again. 6th Stop: Pure Space again. 7th Stop: Pure Time again.
8th Stop: Oh, hey, light space and time are all re-combing in a new and unique way...Kinda cool. I can hang out here, learn a few things, though "here" feels fundamentally different from everywhere else. I'm a little thirsty, after all, this travel across the seam, so maybe a little water...and why is the water flowing up my throat? Definitely got to run some tests here to learn the new laws of physics.
This pattern would continue to repeat ad infinitum. The picture below might help visualize what I'm describing. Only, in addition to the one yellow strand (light), it also has a blue (space) and a green one (time). 781