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.
I'm not sure how popular my answer will be, but I think that in many cases colonialism has indeed been a precursor to development in previously colonized countries. In India, for example, the colonizing British created what amounted to a governing class of educated Indians. I would expect that this same class went on to govern the country when India gained its independence. Of course, I don't think that colonialism is a necessary and desirable precursor to development, and I agree with the other posters who talk about the immense waste of money and talent for both the colonizer and colonized.
In addition to Japan, as discussed by pohnpei397, I would encourage you to consider searching for the terms "Costa Rican exceptionalism." Costa Rica was a Spanish colony, but emerged -- unlike most or all of the other Spanish colonies in Central America -- with a stable democracy, strong economy, good record on human rights, etc.
As I look at the question, I think you are asking whether a country can develop without having been a colony. If you are looking for proof of this idea, I think the best place to look is Japan. After all, it is the only country I can think of (outside of Europe) that has developed into an economic power without having been a colony.
It would be very hard to prove that a country can develop without having been a colony in the way the first answer suggests. For example, it is not at all hard to argue that the US's development was based on a foundation that had been laid when it was a colony.
Japan, on the other hand, was never colonized and was never even a quasi-colony like China. Even so, it developed from an isolated country to a modern power between the 1850s and the early 1900s when it was able to defeat Russia in a war.
I think that you can see many examples of nation building without colonialism. For example, the modern Indian nation is an example of emergence without colonialism. In examining it, one finds that the basis of the modern state was through economic development and not a political one via colonialism. There might be an argument of economic colonialism, that nations are dependent on India such as nations being dependent on a parent nation in colonialism, but in the final analysis India has emerged without colonies and without a sense of political domination over other nations. The focus on economic development preceded the political one via colonialism.
Colonialism refers to the the control of one country or economy - the colony, by another dominant country the coloniser, for the purpose of exploiting resources and markets of the colony by the coloniser. In the short-term colonialism may give a boost to the development of the coloniser at the cost of development of the colony. However colonisation never results in overall development of both the colony and the coloniser. The reason for this is simple. The forcible domination of the colony by the coloniser is necessary only because they stand to loose by arrangements enforced by coloniser. In an arrangement when both parties stand to gain from a relationship, they can work on cooperative basis. Then there is no need for colonization. The fact that voluntary cooperation between countries is much better method of development is amply established by rapid progress of many countries after they obtained freedom from their colonial masters like UK and France. It is worthwhile noting that the erstwhile colonisers have grown at faster rate after giving up control of their colonies. Another major proof of colonisation being a hindrance rather than a help in development is the spectacular development of USA after it secured Independence from colonial control of UK.