Hadith: On the day the Messenger of Allah (may Allah's peace and blessings be upon him) entered Madīnah, everything therein became illuminated. Then on the day he passed away, everything therein became dark

عن أنس بن مالك رضي الله عنه ، قال: «لمَّا كان اليوم الذي دَخَل فيه رسول الله صلى الله عليه وسلم المدينةَ أضاءَ منها كلُّ شيء، فلمَّا كان اليوم الذي مات فيه أَظْلَم منها كلُّ شيء، وما نَفَضْنا عن رسول الله صلى الله عليه وسلم الأيْدِي وإنَّا لفي دَفْنِه حتى أَنْكَرْنا قلوبَنا».
[صحيح] - [رواه الترمذي وابن ماجه وأحمد]

Anas ibn Mālik (may Allah be pleased with him) reported: On the day the Messenger of Allah (may Allah's peace and blessings be upon him) entered Madīnah, everything therein became illuminated. Then on the day he passed away, everything therein became dark. We had not even shaken the dust off our hands after burying the Messenger of Allah (may Allah's peace and blessings be upon him) and we already began to feel the change in our hearts.
[Authentic hadith] - [Narrated by Ibn Majah - Narrated by At-Termedhy - Narrated by Ahmad]

Explanation

Anas ibn Mālik (may Allah be pleased with him) explains that when the Prophet (may Allah's peace and blessings be upon him) entered Madīnah as an immigrant from Makkah, everything in Madīnah started shining with light. When he died, everything started getting dark therein. The light and darkness are metaphorical. When the Companions were done burying the Prophet (may Allah's peace and blessings be upon him) they did not find their hearts in the same state which they had experienced during the Prophet's life. The light, compassion, and amicability receded because the divine revelation had stopped, and they were no longer blessed with the Prophet's Companionship.