Agarose gels (hydrogels) are commonly used in molecular biology for electrophoresis of DNA.
When a solution of hot agarose is poured, the material is clear:
When a gel forms, the material becomes partially opaque:
This phenomenon is NOT observed in acrylamide gels used for electrophoresis of protein. Unpolymerized and polymerized acrylamide both appear clear.
I assume that what we are observing here is light scattering. I am not sure if the scattering is Rayleigh or Mie, and I am not sure why it occurs upon agarose gel formation. I am not sure why it doesn't happen with acrylamide, another gel.