Final answer:
Most supernova remnants have been discovered in the X-ray wavelength region, where the high-energy environment of the remnants can be extensively studied, as in the case of the SN 1006 remnant and the Vela Supernova Remnant.
Step-by-step explanation:
Most supernova remnants have been discovered by observing in the X-ray wavelength region. The high-energy environment of a supernova remnant is best studied using X-rays because these remnants emit strongly in this part of the spectrum. This was beautifully captured, for example, in the composite view of the SN 1006 remnant by the Chandra X-Ray Observatory, which shows the X-rays emanating from the remnant in blue. Observing these sources in X-rays allows astronomers to see details that aren't visible at other wavelengths because the remnants often contain very hot gases with temperatures of millions of K.
Images such as the one provided from the Kepler Supernova Remnant also highlight emission across a range of wavelengths including X-rays, visible light, and infrared. These multi-wavelength studies are crucial to obtain a detailed understanding of the distribution of elements like iron which are produced in the supernova. When supernova remnants collide with the interstellar medium, they heat up the gas to temperatures high enough to emit X-rays, such as the case with the Vela Supernova Remnant.