The swift rally in stock markets this year caught everyone's attention. But with far less fanfare, a frenzy has been taking place in the market for corporate bonds. When credit markets practically shut down last year, businesses had to pay huge premiums to raise money from investors, offering returns of 10 to 20 percent to anyone who would buy a company's debt. Now, investors are the ones paying higher prices as they race back into the bond markets, where companies and governments go to raise money for new projects and mergers, and to finance their daily operations. Some see this as good news, a sign that credit markets are humming along after their near-collapse last year. Others believe Wall Street's next big bubble is inflating as investors chase down returns, and they warn that the big spike in bond prices could roll backward if the economy stumbles and investors again run for safer ground. Some $265 billion has flowed into bond funds since the beginning of the year, according to figures from the Investment Company Institute, 15 times the money that has entered equity funds. A wide gauge of the market for junk bonds is up nearly 50 percent for the year, compared with a 13.5 percent yearly return for the Standard & Poor's 500-stock index. The risk premium on junk bonds over Treasuries -- called the spread -- narrowed to 7.5 percentage points last week from more than 16 percentage points at the start of the year, according to Standard & Poor's figures. In the last three months, yields on the Treasury's benchmark 10-year note dropped from 3.54 percent to 3.22 percent, and the yield on the 30-year 'long bond' fell below 4 percent last week for the first time since April. This means the government pays less to finance its record borrowing, but the bigger demand for Treasuries suggests investors are getting a bit more nervous. Bond yields are still higher than their historic averages, suggesting that there is still more room for profit. But analysts say much of the easy money has already been made. 'I use a golf analogy,' said Mark Kiesel, a managing director at Pimco. 'At the start of the year, we were on a golf course where there was no rough and the fairways were wide open.' Now, he said, the fairways have narrowed. 