What has no content, yet you can see it?

“A window”

A window has content (like glass) and is not empty; it allows you to see through it but does not fit the description of having no content.

“A mirror”

A mirror reflects images and therefore has content (the images of whatever is in front of it), which contradicts the riddle's premise.

“A shadow”

A shadow may appear to be empty, but it is the absence of light and still represents something; it does not align with the concept of having 'no content' as clearly as a hole does.