I want to ask that is it Halal or Haram to go into a unknown wedding without invitation?
No, it is haram to attend a wedding (walima) or other party to which you have not been invited.
يا أيها الذين آمنوا لا تدخلوا بيوت النبي إلا أن يؤذن لكم إلى طعام غير ناظرين إناه ولكن إذا دعيتم فادخلوا
O you who have believed, do not enter the houses of the Prophet except when you are permitted for a meal, without awaiting its readiness. But when you are invited, then enter;
ومن دخل على غير دعوة دخل سارقا، وخرج مغيرا
He who enters without invitation enters as a thief and goes out as a raider.
Note that this is the ruling for the base case; it is permitted when the host has declared a general invitation to all people, or when he has allowed an uninvited guest or when when it is implicitly known that he consents to it.
صرح المالكية والشافعية والحنابلة - وهو المتبادر من أقوال الحنفية - أن حضور طعام الغير بغير دعوة، وبغير علم رضاه حرام، بل يفسق به إن تكرر
The Malikis, Shafi'is and Hanbalis have declared - and it is also understood from the sayings of the Hanafis - that attending a banquet without invitation and without knowing the consent of the host, is haram and one who does so repeatedly becomes a fasiq
In Saudi Arabia, I have attended major weddings involving families that were not known to me without a prior invitation. From this experience, it seems that it would not therefore be haram otherwise I would have been strongly advised against it by Saudi friends and colleagues.
In regard to UmH's answer, however, I should specify that the weddings were not inside a house.