Is it true that there is a name of Allah, we don't know about? Is it supposed to be the 100 name?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu`alayhi wa sallam, is His slave and Messenger.

The Beautiful Names of Allah Almighty are not limited to ninety-nine names, rather there are many other names apart from these that only Allah Knows. Some of them are known to His creation, and there are other Beautiful Names of Allah that He has kept with Him in the knowledge of the Unseen, and which none of His creation has come to know. Rather, the ninety-nine mentioned in the Hadeeth means that whoever learns them will enter Paradise, and the evidence for this is what Ahmed narrated in his Musnad from the Hadeeth of Ibn Mas'oud that the Prophet, sallallaahu`alayhi wa sallam, said: “There is no one who is afflicted by distress and grief, and says: ‘Allahumma inni ‘abduka ibn ‘abdika ibn amatika naasyati bi yadika, maadin fiyya hukmuka, ‘adlun fiyya qadaa-uka. As-aluka bi kulli ismin huwa laka sammayta bihi nafsaka aw anzaltahu fi kitabika aw ‘allamtahu ahadan min khalqika aw istatharta bihi fi ‘ilm il-ghayb ‘indaka an taj’al al-Qurana rabee’a qalbi wa noora sadri wa jala-a huzni wa dhahaba hammi (O Allah, I am Your slave, son of Your slave, son of Your maidservant; my forelock is in Your hand, Your command over me is forever executed and Your decree over me is just. I ask You by every name belonging to You which You have named Yourself with, or revealed in Your Book, or You taught to any of Your creation, or You have preserved in the knowledge of the Unseen with You, that You make the Quran the life of my heart and the light of my breast, and a departure for my sorrow and a release for my anxiety),’ but Allah will take away his distress and grief, and replace it with joy.” He was asked: “O Messenger of Allah, should we learn this?” He said: “Of course; everyone who hears it should learn it.” Classed as Saheeh by al-Albani .

Allah knows best.