When the sun illuminates the back of your subject, it is backlit. When your camera meters for a backlit subject or object, it is much darker than the rest of the scene. You can deal with a backlit object in a couple of ways:
Use exposure compensation to increase the exposure. This option brightens the entire image, which results in some of the image being overexposed. If the natural bright vignette is not a problem, this is an ideal solution.
Bounce some light toward your subject. Use a makeshift or commercial reflector to bounce light toward your subject. This solution won’t work if you’re photographing an animal. The mere presence of the reflector will spook the animal.
Use a Better Beamer on faraway subjects. This is a flash extender you can use with an auxiliary flash unit while using a telephoto lens. The Better Beamer has a Fresnel lens at the end of an extension, which magnifies the power of the flash and causes it to illuminate objects at greater distances. Bird photographers use it to photograph nesting birds.
Use exposure compensation to underexpose the image. This option puts your subject in silhouette. When creating a silhouette, it’s okay to underexpose the picture. Just make sure you have nothing in the background that is large and will also be a silhouette. You don't want it to compete with your subject for the viewer’s attention.