This object is not a basic question. In fact the dome itself shouldnt be there. So for me the basic question is why this dome hasnt been destroyed to expose the dome worshipers and others who think this dome is some holy symbol of islam.
The reason why it has not been demolished
The scholars have explained the shar’i rulings concerning the building of this dome and its obvious effects on the followers of bid’ah’ who have developed an attachment to this structure and its colour, and they praise and venerate it a great deal in their poetry and writings. Now it is up to the authorities to implement these fatwas, and this is nothing to do with the scholars.
The reason why it is not demolished is so as to ward off fitnah and for fear that it may lead to chaos among the ordinary people and the ignorant. Unfortunately the ordinary people have only reached this level of veneration towards this dome because of the leadership of misguided scholars and imams of innovation. They are the ones who incite the ordinary people against the land of the Two Holy Sanctuaries and its ‘aqeedah and its manhaj. They are very upset about many actions that are in accordance with sharee’ah in our view and contrary to innovation in their view.
Whatever the case, the shar’i ruling is quite clear, and the fact that this dome has not been demolished does not mean that it is permissible to build it or any dome over any grave, no matter what grave it is.
Shaykh Saalih al-‘Usaymi (Pbuh) said: The fact that this dome has remained for eight centuries does not mean that it has become permissible, and being silent about it does not indicate approval of it or that it is permissible. Rather the Muslim authorities should remove it and put it back as it was at the time of the Prophet (peace and blessings of Allaah be upon him). They should remove the dome and the adornments and engravings that are found in the mosques, above all the Prophet’s Mosque, so long as that will not lead to an even greater fitnah. If it would lead to an even greater fitnah, then the ruler should postpone the matter until he finds an opportunity for that. [Bida’ al-Quboor, Anwaa’uha wa ahkaamuha (p. 253)]