What can light pass through clearly?

Answer

Your question is somewhat difficult to interpret, but presumably you are referring to visible light and the word "clearly" is key. Many media are transparent to the electromagnetic radiation we call light. Light passes through a vacuum freely, without obstruction or change. A wide range of gasses and solids transmit light (permit light to pass through) in varying degrees. Clear glass is highly transparent. Gold hammered to a thickness of a few atoms is translucent. These denser media cause temporary or permanent change to light waves passing through them. Glass causes the light to slow down slightly so that a wave front striking a flat clear glass surface obliquely is bent. Upon exiting a parallel surface into its original, less dense medium, it resumes its original speed and direction. White light is made up of a continuous spectrum of wavelengths and corresponding frequencies that we see as a rainbow of colors. White light passing into a thin sheet of gold is diffused (scattered), and while all wavelengths are absorbed to some degree, only a small amount of green light actually emerges. Light is affected by any medium other than vacuum by slowing, absorption in all or some wavelengths, and diffusion. All media denser than vacuum slow and absorb, but diffusion is a function of the clarity of the material.

An earlier answer said light passes through darkness. Somewhat poetic, perhaps, but not quite accurate. Darkness is definable as the absence of light, but if you consider light in a broader definition encompassing the entire electromagnetic spectrum, from low frequency long radio waves to very high frequency short waves like X-rays and gamma rays, you would be hard pressed to find a truly dark place anywhere in the Universe, if only you have the means to see.