The Qur’an says that the contract of Nikah (marriage) should be undertaken in the manner that is recognized, supported and followed by the honorable and noble people of a society (see: Al-Baqarah 2: 232).

In the light of the guidance provided by the Qur’an, it may be said that a nikah, to be valid, must fulfill three necessary conditions; firstly, it must entail the free consent of both the male and the female entering into this contract; secondly, it (the contract) must not be kept hidden and should be declared in the society and thirdly,payment of mehr(the obligatory token money paid by the bridegroom to ensure financial responsibility )

The first condition is quite obvious. It is the very spirit of every contract that it should be based on the free consent of all the parties entering into that contract. Any contract, which is not based on the free consent of the parties concerned is void in the eyes of the law.

Furthermore, because nikah (marriage) is a social contract, it must, therefore, be declared and recognized in the society. It is basically this social recognition, which nikah provides to the mutual bond between a male and a female who may otherwise be unrelated to each other. If you look closely at the marital relationship between a man and a woman, you shall see that the only difference it has when it is compared to an illegitimate relationship between a man and a woman who are not married to each other is that the latter is not recognized by the society to be any relationship at all. Every social relationship, by its very nature, gives all the parties in such a relationship some rights on the others and imposes on them some duties toward others. A nikah is basically a social declaration by both the parties of the intent of fulfilling all the rights and duties that arise out of wedlock. If any nikah fails to fulfill this condition of social declaration, it can then, by no means, be termed as a nikah.

Answer taken from : https://uiuk.org/is-a-secret-nikah-valid/