MN 123 Acchariy'abbhutadhamma: Wonderful and Marvellous
“When the bodhisatta left his mother’s womb, a great, measureless radiance surpassing the effulgence of the devas appeared in the cosmos with its devas, Māras, & Brahmās, with its people with their contemplatives & brahmans, their royalty & commonfolk. Even in the inter-cosmic void of the unrestrained darkness, pitch-black darkness, where even the light of the sun & moon—so mighty, so powerful—doesn’t reach, even there the great, measureless radiance surpassing the effulgence of the devas appeared. And the beings reborn there perceived one another by that radiance: “So other beings have been reborn here, too!” And so this ten-thousand-fold cosmos shivered & quivered & quaked, while the great, measureless radiance appeared in the cosmos, surpassing the effulgence of the devas.”
This great light appears when a Buddha is born, gets enlightened, starts teaching and dies. I find it so moving that the beings in outer darkness can see by it enough to realise that they are not in fact alone and thus start their journey back upwards.