The Magellanic Stream, which extends almost halfway around the Milky Way, is only detectable at radio wavelengths, and made visible in the wonderful composite image above.
Credit: Science – NASA, ESA, A. Fox, P. Richter et al.
Image – D. Nidever et al., NRAO/AUI/NSF, A. Mellinger, LAB Survey, Parkes, Westerbork, and Arecibo Obs.