Both makzimus and you seem to add a letter to the wrong "word" because the original words are rabb رب and al-alameen العالمين (alamin) while fatha, dama, kasrah are just nuances for the pronunciation: that means writing rabbil alamin is wrong as it is like if you have token the lam ل from al-alameen and added it to rabb ربل and ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.