What constitutes the expanding supernova remnant (SNR) cloud formed after explosion?

The star's outer layers mixed with the interstellar medium (ISM)

The supernova explosion is characterized by two primary outcomes: the dense, centralized core remnant and the vast, dispersing shell of material. This expanding shell is specifically termed the supernova remnant (SNR). This remnant is not merely the star's expelled debris; it is the star's outer layers, blasted away at high velocities, which then sweep up and interact with the existing interstellar medium (ISM)—the cold gas and dust already present in the galaxy. This mixture creates a complex cloud that glows visibly across multiple wavelengths, including radio waves, X-rays, and visible light, as the outward rush of material compresses and heats the surrounding environment over thousands of years.