Why is Israel believed to have nuclear weapons why isn't it confirmed? Where's the evidence they do?

For one, they're not a part of any treaty that prohibits the building of nuclear weapons.

A treaty of such magnitude exists, called the Treaty on the Non-Proliferation of Nuclear Weapons. Basically, the treaty says that nations can gain the technology and training for the use of nuclear technology from those that use nuclear technologies in exchange for using them for peaceful purposes. In other words, anyone that is a part of this treaty is allowed to build nuclear power plants to supply power for their country, not to build nukes.

Last time I checked, Israel is one of four countries that is not a part of this treaty. So, even if Israel did have nuclear weapons (technically, as I'll explain later, no one can say for sure, but for all practical purposes, it does), it's not as though they violated any rules, technically.