Gfyuki

Around 100 million years ago, the Solar System was instantaneously encased in a massless, magical sphere centered about the Sun. The boundary is about 10,000 AU (.158 ly) in radius and behaves abnormally: matter and energy from outside the boundary may enter into the Solar System unaffected, while matter and energy trying to exit is essentially erased (if you were to reach your hand beyond the boundary, you'd retract a gory stump).

Question: Assuming that everything else proceeded as usual with the dinosaurs' extinction and humanity's uprising... With such a structure just thrown up like that in the distant past, could there be any astronomically glaring signs or repercussions of it that would tip us (present-day "modern" astronomers with our space telescopes and space probes) off to its existence?

I am searching for any consequential phenomena that results from the boundary's introduction that also signals to modern astronomers that something is at least not right with outer space at that distance (keep in mind, the alien sphere itself is massless, essentially transparent, and not a blackbody (matter and energy erased is not absorbed and re-emitted)). I feel like this question is better posed under the yes-or-no format. So, if the side-effects of such an alien boundary are too little for modern (can be any era up to modern, really) astronomers to detect, or there are no side-effects, then showing that with a science-based analysis constitutes a "no" answer. Showing that some form of resultant phenomena exists that also falls under astronomers' threshold of detection constitutes a "yes" answer.

Potential pointers:

(These are just some things I've contemplated during my research.)

Of course, astronomers won't be able to see the boundary directly, as light from the outside simply passes through it unaffected in any way, though, they may be able to infer its existence somehow.

Not many striking things seem to orbit 10,000 AU from the Sun. The farthest object we've currently discovered, Farout, orbits about 120 AU out. The Oort Cloud, however, is a different story. The Oort Cloud is a hypothetical structure which defines the Sun's cosmographical Hill sphere, the region within which objects have the potential to orbit the Sun. Its radius ranges from 2,000 to 200,000 AU, so the alien boundary would have intersected and partitioned it. 100 million years is quite a few Earth-orbits, even for those super-distant objects with multi-thousand-year years, so perhaps modern astronomers would see a deficit of long-period comets with aphelia greater than 10,000 AU. (Perhaps a detectable discrepancy?)

Scholz's Star, WISE designation WISE 0720−0846, is a red dwarf that has been modeled to have passed through the Oort Cloud of the Solar System at a distance of around 52,000 AU, around 70,000 years ago. Similarly, Gliese 710 or HIP 89825 is predicted to have a close approach with the Sun at a distance as near as 13,300 AU (just outside the alien boundary) within the next 15 million years. The Wiki's source states that there is a 1 in 10,000 chance that the star penetrates less than 1,000 AU, significantly perturbing Kuiper belt objects. According to this paper, stellar approaches closer than around 50,000 AU happen about every 9 million years, with probabilities of even closer approaches.

Exoasteroids and exocomets, such as 'Oumuamua, will have entered the Solar System, though, in the ~1,800 years it will take to reach the boundary (1.496e+12 km / 26.3 km-per-sec / 60 seconds-per-min / 60 minutes-per-hour / 24 hours-per-day / 365.25 days-per-year = 1,802 years), we won't see it or others like it leave. (We may, however, see captured exosolar bodies.) Any future endeavors to send probes or spacecraft to other star systems, like Breakthrough Starshot or Project Daedalus, will not work because they simply cannot penetrate the boundary, so, after the first few of these attempts, we will begin to at least suspect something.

Yes. Astronomers could see the barrier directly because the barrier would emit Hawking radiation.

Pairs of particles and antiparticles are constantly appearing and disappearing all over the place throughout space. This is called quantum fluctuation. It's usually hard to detect quantum fluctuation because the particle pairs annihilate each other soon after forming. If one of them is is removed by, say, falling into a black hole or getting annihilated by your barrier, and the sister particle doesn't get annihilated then the sister particle gets to do something else like becoming visible to astronomers. In the case of black holes these escaping particles are called Hawking radiation.