"it is the largest known structure in the universe"
Why did it stop at a planck length? I thought space was continuous.
That is actually an unknown. There are many theories that postulate discrete space, but there has been a dearth of reliable experimental tests for it.
It's a hard hypothesis to prove either way.
One finds that quantum effects dominate the physics at smaller length scales.
In quantum theory, the smaller a thing is (its Compton wavelength) the higher its mass or energy. At the Planck scale the density becomes large enough for a black hole to form. So we can get quantum fluctuations creating virtual black holes, which means that the description of spacetime as a continuous thing is invalid.