Something dirt comes out of skin layer which is naturally a deadskin cell or something else does it prevent reaching skin ,my ghusl is still valid or not? and every one may have this dirt at there back of the body
The skin that needs to be reached by water during ghusl is the visible, superficial part. Note that the prophet poured three times water over his head to perform ghusl so you shouldn't waste your time nor water for performing it better than necessary.
Jabir bin `Abdullah said to me, "Your cousin (Hasan bin Muhammad bin Al-Hanafiya) came to me and asked about the bath of Janaba. I replied, 'The Prophet (ﷺ) uses to take three handfuls of water, pour them on his head and then pour more water over his body.' Al-Hasan said to me, 'I am a hairy man.' I replied, 'The Prophet (ﷺ) had more hair than you'. " (See for example in Sahih al-Bukhari)
The qur'an itself only say wash yourselves which means all your body.
... And if you are in a state of janabah, then purify yourselves. ... (5:6)
Scholars made the matter more difficult by discussing body parts that must be taken into account such as rinsing mouth and nose etc.. While it could be more simple
There's no need to worry about the validity of your ghusl if you have your best to make water reach all your body and performed wudu' during or after the ghusl -while being wet with the water of ghusl-.
If you felt this "dirt" creating doubts you may use tools to reach it or ask a mahram of yours for help while performing ghusl. This is common in the maliki madhhab as a requirement there is to somewhat scrub your skin.