There are known attacks against full AES. Using Biclique Cryptanalysis, full AES with a 128 bit key has been reduced to 126 bits of entropy, which seems to be the best result for key recovery attacks so far. The key space is, however, still impractical to search. At 1015 operations per second, which is the approximate computing power of a very large and expensive supercomputing cluster today, to find a key would still require an average time of 8.5 × 1022 seconds (2126 operations/1015 operations per second), or about 2.7 quadrillion years. The only known attacks against full AES also has the tiny problem of requiring petabytes of disk space. All in all, not very practical.
The main reason it is stronger than it's predecessor, the DES cipher, is due to it's larger key size. The fact that it is newer also reduce the window that cryptanalysts have had to come up with various attacks. Properly implemented AES has no known, practical attacks.