Estimate the wavelength of an oscillatory signal
Consider the code snippet below to find the three peaks in the graph. Note, that peak search usually only searches for positive peaks. Hence, I negated the signal to search for the down-peaks. Also, I adapt the SNR threshold in the code to detect only the very strong peaks.

import urllib2
import scipy.signal as signal

url = 'https://raw.githubusercontent.com/Omer80/wavelength/master/oscillations.dat'
response = urllib2.urlopen(url)
cr = np.loadtxt(response)
X = cr[0,:]
Y = cr[1,:]

plt.subplot(2,2,1)
plt.plot(Y)

plt.subplot(2,2,2)
plt.plot(X, Y)

peakind = signal.find_peaks_cwt(-Y, np.array([100]), min_snr=5)
peakind = np.array(peakind)
print peakind


plt.stem(X[peakind], 1+0*peakind);

period = int(np.mean(np.diff(peakind)))
print "Period between peaks:", X[period]-X[0]


Output: 

[1919 2500 3080]
Period between peaks: 0.145036259065