河外背景光（英语：extragalactic background light，EBL）是由于恒星形成过程加上活跃星系核（AGNs）的活动累积在宇宙中形成的弥漫性辐射。这种辐射涵盖的波长范围在0.1～1000微米（这些是紫外线、光学、和红外电磁频谱的范围）。河外背景光被定义为涵盖整体电磁频谱的弥漫河外背景辐射（DEBRA，diffuse extragalactic background radiation）的一部分，继宇宙微波背景辐射之后，河外背景光是第二高的弥漫背景能量，因而成为了解宇宙能量平衡必要的条件。

The diffuse extragalactic background light (EBL) is all the accumulated radiation in the universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers the wavelength range between ~0.1 and 1000 µm (these are the ultraviolet, optical, and infrared regions of the electromagnetic spectrum). The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the overall electromagnetic spectrum. After the cosmic microwave background, the EBL produces the second-most energetic diffuse background, thus being essential for understanding the full energy balance of the universe.