Can anyone please give me a proof that that colonialism is not necessary for development? Maybe anyone can explain or at least give me a website or two.
If we define "colonialism" as one group expanding into another group's territory with the intent to control it and the native inhabitants, you've got, unfortunately, a good summary of human history. Even countries that we would today consider homogeneous were at one time composed of several smaller political and economic entities that were either conquered outright or made dependent upon another region (consider early medaeval England or France, for example.) If we define "development" as a rising standard of living, then it appears that most countries ascribe to improving their society and infrastructure to achieve that end. Colonizing powers, by definition, have some degree of political, social, and economic organization that allows them to expand to other regions; chiefly this is to acquire raw resources that it may be lacking. "Developing" countries may have the resources, but not the expertise to fully exploit them -- so they seek (or are offered "help" from) colonizing countries. Sadly, no matter what historical era you study, you will find this pattern again and again. In short, you never get a "freely-developing" country that isn't under the auspices of some greater power. Inasmuch as one country's influence over another brings mutual benefits, colonialism becomes necessary for development.
The idea of "development," which is part and parcel of the great Western love of progress as manifested in the great colonial impulse which erupted in the previous centuries and is still somewhat active today, is already a very "colonizer-centric" mindset.
Your real question has to do with what constitutes development and progress. If you have a homogeneous view of "development" (technological progress, global market strength) it's going to be difficult for you to find "developed" societies which were not once under the rule of a colonial power or a great power themselves.
Thinking of the so-called BRIC countries which are "developing" now, you have Brazil and India, which were once colonized, and India and China, who are colonial agents in themselves. As I said before, it will be difficult, given a homogenous definition of development to find such a culture that is not either colonizer or colonizee.
When you asked this question in the Q and A section, we were not clear on what you meant. Are you asking for proof that a country can develop even if it wasn't a colony? Or are you asking for proof that a country can develop even if it has no colonies?